When text on the screen is “active”, so that it does something special in response to mouse clicks or RET, it often has associated help text. For instance, most parts of the mode line have help text. On terminals that support mouse tracking, Emacs displays the help text as a “tooltip” (sometimes known as “balloon help”) or in the echo area, whenever you leave the mouse stationary over the active text. See Tooltips.
If your terminal doesn't support mouse-tracking, you can display the
help text for active buffer text using the keyboard. C-h .
display-local-help) displays any help text associated with the
character after point, using the echo area. To display help text
automatically whenever it is available on the character after point, set