When point is idle for more
than reftex-idle-time seconds on the argument of a
\ref macro, the echo area will display some
information about the label referenced there. Note that the
information is only displayed if the echo area is not occupied by
a different message.
RefTeX can also display the label definition
corresponding to a \ref macro, or all reference
locations corresponding to a \label macro. See
Viewing
Cross-References, for more information.