Ardour follows the conventions used by most other computer software (including other DAWs) for selecting objects in the GUI.
Clicking on object (sometimes in a particular part of its on-screen representation) will select the object, and deselect other similar objects.
click on an object toggles it selected status, so using click 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 click one object and then Shitft click another to select both of them as well as all object in between them.
These basic principles apply to Range Selection - click-drag to select a range, drag to create multiple ranges and shift-click to extend a range to cover a wider area.