\label{key}
A \label command appearing in ordinary text assigns to the key the number of the current sectional unit; one appearing inside a numbered environment assigns that number to the key.
A key can consist of any sequence of letters, digits, or punctuation characters. Upper- and lowercase letters are different.
The number assigned to the key can be accessed by use of the \ref command. For example,
\begin{equation} .... .... \label{eq:no1} \end{equation} ... One can see from Eq.~(\ref{eq:no1}) that ...
In a table or figure environment, the \label command should follow the \caption command (or be part of its argument) since it is the \caption command which generates the number.