In some cases, we want to apply an operation to the entire area of the canvas EXCEPT one part. This is common when we want to make most of the canvas transparent, or fill it mostly with one color, or apply a filter to most of it. In these cases, it's faster to select the part that we don't want to modify, and then "invert" the selection so that non-selected pixels become selected and vice-versa.
To do this in Krita, click on Select -> Invert Selection on the menubar (keyboard shortcut: Ctrl+Shift+I).
Note: in some image editors, the keyboard shortcut is Ctrl+I instead. In Krita, that shortcut means "invert the colors of the image" instead.