Tooltips are small windows that display text information at the current mouse position. They activate when there is a pause in mouse movement. There are two types of tooltip: help tooltips and GUD tooltips.
Help tooltips typically display over text—including the mode line—but are also available for other parts of the Emacs frame, such as the tool bar and menu items.
You can toggle display of help tooltips (Tooltip mode) with the command M-x tooltip-mode. When Tooltip mode is disabled, the help text is displayed in the echo area instead.
GUD tooltips show values of variables. They are useful when you are debugging a program. See Debugger Operation.
tooltip-delay specifies how long Emacs should
wait before displaying a tooltip. For additional customization
options for displaying tooltips, use M-x customize-group
<RET> tooltip <RET>. See X Resources, for information on
customizing the windows that display tooltips.