Ardour follows the conventions used by most other computer software (including other DAWs) for selecting objects in the GUI.
Clicking on an object (sometimes on a particular part of its on-screen representation) will select the object, and deselect other similar objects.
A left click on an object toggles its selected status, so using left on a series of objects will select (or deselect) each one of them. You can construct completely arbitrary selections with this technique.
In cases where the idea of "select all objects between this one and that one" makes sense, you can select one object and then click left on another to select both of them as well as all objects in between.
To select a time range in the Editor, click Left and drag the mouse. A Left drag then lets you create other ranges and a left click extends a range to cover a wider area.
The set of objects (including time range) that are selected at any one time is known as the selection. Each time you select or deselect an object, the new selection is stored in an undo/redo stack. This stack is cleared each time the content of the timeline changes. If you have built up a complex selection and then accidentally cleared it, choosing Edit > Undo Selection Change will restore your previous selection. If you then decide that you had in fact made the correct change, choosing Edit > Redo Selection Change will take you back to where you were before you chose Edit > Undo Selection Change.