Tooltips are a GUI (Graphical User Interface) element which typically pops up when the mouse pointer is hovering over an item in the GUI and provides some contextual information or clarification.
A tooltip is a small message, helping to understand the underlying content, often a button, a selection or link. It often appears after a certain amount of time, usually a few seconds, or while hovering over a certain area, and vanishes easily in order to not disturb too much.
Some visual examples of tooltips:
This video presents various flavors of tooltips.