Select all
Select all is selecting all text, files, or other objects currently listed or displayed. In most programs, pressing Ctrl+A selects (highlights) everything in the current window.
For example, while in Microsoft Word, if you press the Ctrl+A keyboard shortcut, all text in the document is selected. In Explorer, pressing Ctrl+A selects all drives, files, and folders in the open location.
Once everything is selected, press and hold down the Ctrl and click with your left mouse button to unselect files. For example, in Explorer, after everything is selected, to remove three files from the selection, press and hold Ctrl, then click each file to unselect.
Also, after everything is selected, you could cut or copy what's selected and then paste it elsewhere or delete everything selected.