In Emacs, stretches of active text (text that does something special in response to mouse clicks or <RET>) often have associated help text. This includes hyperlinks in Emacs buffers, as well as parts of the mode line. On graphical displays, as well as some text terminals which support mouse tracking, moving the mouse over the active text displays the help text as a tooltip. See Tooltips.
On terminals that don't support mouse-tracking, you can display the
help text for active buffer text at point by typing C-h .
display-local-help). This shows the help text in the echo
area. To display help text automatically whenever it is available at
point, set the variable