Snap will cause region drags and other mouse-driven operations to jump to positions determined by the nearest snap setting when the mouse is close enough to this snap point. Snapping is sometimes (improperly) referred to as magnetism.
The Snap options are set in the Preferences. Those include the Snap Threshold which determines how close the mouse has to be to a snap anchor to induce a snap, and the snap anchors themselves, among:
The Grid helps visually placing items in time. It will draw lines at selected intervals as chosen in the drop-down selector. Musical grid settings (Bar to Sextuplets) obviously depend on the tempo and time signature, so changing the tempo or time signature will rescale the grid, while absolute grid settings (Timecode, etc…) won't be affected.
A word about time signature in this context: a time signature consists of 2 numbers. The upper one determines how many beats are in a bar, the lower one, what division of a note a beat represents (e.g. : 4 stands for a quarter note). At e.g. 80 bpm, one beat lasts 1/80th min, so 0.75 sec. If the time signature is 3/4, there are 3 beats in a bar so a bar lasts for 3 × 0.75 = 2.25 sec. Choosing 1/8 Note as the grid setting will draw grid lines every 0.75 ÷ 1/4 × 1/8 = 0.375 sec.
The grid density can be either based on musical time:
Or absolute time:
Alternatively, Ardour can disable the usual musical/non-musical time grid and make the current playhead position the only snappable target. To do that, select the "Playhead" item in the drop-down list.
The grid consist of lines running vertically in the edit canvas. When zooming too far out, the grid can become too coarse. Ardour tries not to show “too many” or “too few” grid lines depending on the zoom level, based on the Approximate Grid/Ruler granularity (pixels) Preferences parameter. As a consequence, when the Grid is in the Snap anchors, it is possible that items snap in-between the grid lines sometimes. That’s expected behavior.
By default, a region's beginning will be used as the reference for both types of snapping, this behaviour can be changed by setting a sync point in the region, by selecting the region(s) and pressing V. This will set the sync point to the current edit point.
Snapping can be temporarily disabled by using a keyboard modifier while editing, by default.
Snapping can also be temporarily set to relative, i.e. snapping will occur relative to the current position of the dragged item. E.g. if the Grid is in the Snap options, and the grid is set to Bars, using this keyboard modifier while dragging will snap at every bar relative to the region's beginning (or sync point) instead of the absolute musical bars.
The keyboard modifiers are defined in the Preferences.