In typesetting math in LaTeX, there is inline mode (math inside text) and display mode (separate equations outside text). This tag is used for inline mode. It should also be used for other structures that provide a similar distinction between its usage, like the {listings} package's environment and inline form.
Math in LaTeX is most commonly set as inline (or text style) mathematics
...here is some text and $<math content>$ and some more text...
or as equations (or display style) mathematics
...here is some text and
\[
<math content>
\]
and some more text...
For specific use on the notation/symbols for setting these, see Why is \[ ... \] preferable to $$ ... $$? and Are \( and \) preferable to dollar signs for math mode?
Other packages may also provide inline variants to their display environments. Most notably listings provides an inline \lstinline version to its display lstlisting environment:
...here is some text and some \lstinline!code!, and some more
\begin{lstlisting}[basicstyle=\ttfamily]
code
\end{lstlisting}
