A fade makes something gradually appear or disappear. It is like a room light slowly turning on or off instead of suddenly flashing.
A photo might fade in after it loads, or a temporary message might fade away after a moment. The gentle change can make the arrival or departure feel less abrupt.
Do not use a fade when people need to act immediately on the content. Keep the effect brief and make sure hidden content is not still taking up space or receiving keyboard focus after it disappears.