The small responses an interface gives to a single action — a button settling, a field validating as you type, a list animating to show what just changed.
They are how an interface tells the user it heard them. Without feedback, people press twice, assume failure, and lose confidence in software that is working correctly.
We design feedback for every state that can take time or fail, keep motion short and interruptible, and respect reduced-motion settings rather than treating animation as decoration.