When point is idle for more than reftex-idle-time seconds on the
argument of a \cite macro, the echo area will display some
information about the article cited there. Note that the information is
only displayed if the echo area is not occupied by a different message.
RefTeX can also display the \bibitem or BibTeX database
entry corresponding to a \cite macro, or all citation locations
corresponding to a \bibitem or BibTeX database entry.
See Viewing Cross-References.