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.