The Edit menu groups together the actions related to the edition, and so will be mostly used while in Editor mode.

Undo (action)
Reverts the last editing operation, namely action
Redo
Does the last editing operation again, after an Undo
Undo Selection Change
Reverts the last selection operation
Redo Selection Change
Does the last selection operation again after an Undo Selection Change
Cut
Deletes the current selection, but puts it in memory ready to be pasted
Copy
Copies the current selection to memory
Paste
Pastes the memory at the edit point, after a Cut or Copy operation
Select
Select All Objects
Selects all the regions and automation pints in the session
Select All Tracks
Selects all the tracks, busses and control masters in the session
Deselect All
Deselects all objects or tracks, nothing is selected
Invert Selection
Select the previously unselected regions, and deselect the previously selected ones
Set Range to Loop Range
Creates a range selection on the selected tracks, based on the selected loop markers, and switches to Range Mode tool
Set Range to Punch Range
Same as above, based on the selected punch markers
Set Range to Selected Regions
Same as above, based on the selected regions (i.e. from the start of the earliest region to the end of the latest one)
Select All After Edit Point
Select all the regions and automation points that exist after the Edit Point, even if the region starts before it. If some tracks are selected, only selects on these tracks.
Select All Before Edit Point
Same as above, but before the Edit point (i.e. to the left of it)
Select All Overlapping Edit Range
Select all the regions and automation points of which at least a part is in the current selection range
Select All Inside Edit Range
Selects all the regions that are completely inside the selection range, i.e. their start and end are inside the range. If some tracks are selected, only selects on these tracks.
Select All in Punch Range
Selects all the regions of which a part in in the punch range. If some tracks are selected, only selects on these tracks.
Select All in Loop Range
Same as above, based on the loop range
Move Range Start to Previous Region Boundary
Extends the left boundary of the range to the left to the next region start or end. The region must be in the range.
Move Range Start to Next Region Boundary
Same as above, to the right (reduces the selection)
Move Range End to Previous Region Boundary
Same as above, with the right edge of the range, to the left (reduces the selection)
Move Range End to Next Region Boundary
Same as above, with the right edge, to the right (extends the selection)
Start Range
Sets the left edge of the range to the Edit point
Finish Range
Sets the right edge of the range to the Edit point
Select Next Track or Bus
Select the track or bus under the currently selected one. If multiple tracks are selected, only the first one is considered
Select Previous Track or Bus
Same as above, with the track/bus above the first one selected.
Delete
Deletes all that is currently selected
Crop
Cuts the parts of the regions that are outside the range boundaries. Only applies on the regions that belong at least in part to the range.
Split/Separate
Cuts the selected regions at the Edit point, separating them in two regions
Separate
Separate Under
Removes all the parts of the regions that are under the selected one. Once done, the selected region is alone on its part of the track.
Separate Using Loop Range
Cuts the selected regions or the regions on the selected tracks along the Loop range's start and end markers. If nothing is selected, acts on all tracks at once.
Separate Using Punch Range
Same as above, with the Punch range markers
Align
Align Start
Moves the selected regions to align the beginning of the regions to the Edit point
Align Start Relative
When multiple regions are selected, moves all the regions together as a block to align the beginning of the earliest one to the Edit point.
Align End
Moves the selected regions to align the end of the regions to the Edit point
Align End Relative
When multiple regions are selected, moves all the regions together as a block to align the end of the latest one to the Edit point.
Align Sync
Moves the selected regions to align the Sync point of the regions to the Edit point
Align Sync Relative
When multiple regions are selected, moves all the regions together as a block to align the earliest Sync point to the Edit point.
Fade
Fade Range Selection
For all the regions that either begin or end in the range, create a fade in or out on the regions length.
Set Fade In Length
If the edit point is withing the region boundaries, adjusts selected audio regions' fade in to end at the edit point.
[] Fade In
Toggles the fade in on the selected region on or off
Set Fade Out Length
Same as above, for the fade out
[] Fade Out
Toggles the fade out on the selected region on or off
Remove Last Capture
Destroy the last recording. A prompt reminds the user this cannot be undone.
Edit point
Change Edit Point
Toggles between the mouse and the playhead as the Edit point
Change Edit Point Including Marker
Toggles between the mouse, the playhead and marker as the Edit point
Snap Mode
() No Grid
Disables snapping, i.e. allows free movement of regions and boundaries
() Grid
Forces snapping, so any move of region boundary will be lined to the grid as chosen
() Magnetic
If the movement of the region or boundary happens near a grid line, snaps, otherwise, allow free movement
Next Snap Mode
Toggles between the No Grid, Grid and Magnetic snap modes
Next Snap Choice
Circles through the snap choices, as detailed below
Previous Snap Choice
Circles through the snap choices, as detailed below, in reverse order
Next Musical Snap Choice
Circles through the musical snap choices, e.g. those expressed in bars and beats
Previous Musical Snap Choice
Same as previous, but in reverse order
Snap To
Snap to CD Frame
The grid unit will be 1/75th of a second
Snap to Timecode Frame/Second/Minute
The grid unit will be based on the timecode settings for the session
Snap to Second/Minute
The grid unit will be based on absolute times
Snap to nth
The grid unit will be 1/n beats and will depend on the tempo and meter in effect at that point in the timeline.
Snap to Beat
Same as above, whole beat
Snap to Bar
Same as above, whole bar
Snap to Mark
The grid will be made of markers
Snap to Region Start
No grid, the regions will snap to the closest region start on any track
Snap to Region End
Same as above with the regions' ends
Snap to Regions Sync
Same as above, with the Sync points (by default, start of the region)
Snap to Region Boundaries
Same as above, for both the starts and ends of regions
Tempo
Set Tempo from Region = Bar
Computes the tempo so that the duration of the first selected region is 1 bar. Ardour prompts if the user wants it to be the global tempo, or a tempo marker at the beginning of the region used
Set Tempo from Edit Range = Bar
Same thing, with the current Range instead of a region
[] Smart Object Mode
Toggles the Smart Mode, allowing the mouse to be in Range Mode in the upper half of a region, and in Grab Mode in the lower half
Scripted Actions
[] Script Manager
Shows the Script manager, allowing to use and manage the Lua scripts in the session
Unset #n
Deactivate the nth script
Preferences
Displays the Preferences panels, allowing to change Ardour's behaviour