To select a file or a clip in PiTiVi, click it.
Multiple selections allow you to apply actions (dragging, deleting, grouping, etc.) to many clips at once.
Selecting multiple adjacent items:
Click the first item.
Hold the Shift key.
Click the last item.
Selecting multiple nonadjacent items:
Hold down the Ctrl key.
Click on all the items you want to select.
You can deselect an individual item by clicking on it again with the Ctrl key held down. This can be used in conjunction with the Selecting multiple adjacent items technique above.