A tooltip is a short hint that appears beside something on a screen when you point at it or move keyboard focus to it. It is like a tiny label that waits nearby until someone needs it.
For example, a small question-mark control might show “Choose a stronger password” when focused. This lets the page stay uncluttered while still explaining less familiar controls.
Keep a tooltip brief and make sure it can also be reached without a mouse. Important instructions should stay visible on the page, because a person may never notice a hidden hint.