\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.
The \label command is fragile but it can be used in the argument of a sectioning command or \caption command.