\cite[text]{key_list}
The key_list argument is a list of citation keys. This command generates an in-text citation to the references associated with the keys in key_list by entries on the aux file read by the \begin{document} command.
\bibitem and \cite are used together much as \label and \ref to produce a correspondence between items in the Reference or Bibliography (thebibliography environment).
The \cite command is fragile.