A selection is a piece of an image you have somehow selected.
Selections work on all image types.
If one or more selections are in effect, GIMP operations should operate only on the selected areas. Undo works on selections as it does other actions. The top two rows of buttons on the main toolbar are all selection buttons. Selecting any of these turns the cursor into a selection tool while over an image. From left to right, then top to bottom, here are the selection functions available on the toolbar.