Animation changes an item's appearance over time to make it seem to move or change. It is like a flipbook: each small change creates the feeling of motion when the pages turn quickly.
A button might gently pulse to show that it is waiting for attention, or a small check mark might appear after saving. Movement can make a change easier to notice.
Use animation to explain something, not just to decorate everything. Too much movement can distract or make people uncomfortable. Keep it brief and allow reduced-motion preferences to be respected.