To select a single item just click on it with the mouse. To perform an operation on multiple files or video filters you must first select them. There are several ways to select multiple items:

Using mouse - Click on the first item you wish to select. Use the scroll bar to find the last item of your selection, hold down the Shift key and click on it. You should see that multiple items are now highlighted. To select all items in the list click Edit > Select All.

Using keyboard - Move to the first item by pressing the arrow keys, then press the Shift key. While holding down the Shift key move in the list using arrow keys, PgUp, PgDn, Home or End until you select all desired files. To select all items in the list just press Ctrl+A.