Multiple plots don't work in TeXShop mac

I was trying to include multiple plots with the following in Texshop in my mac:

\begin{figure}
    \centering
    \begin{subfigure}[b]{0.3\textwidth}
        \centering
        \includegraphics[width=\textwidth]{l1.jpg}
        \caption{$y=x$}
        \label{fig:y equals x}
    \end{subfigure}
    \hfill
    \begin{subfigure}[b]{0.3\textwidth}
        \centering
        \includegraphics[width=\textwidth]{l1.jpg}
        \caption{$y=3sinx$}
        \label{fig:three sin x}
    \end{subfigure}
    \hfill
    \begin{subfigure}[b]{0.3\textwidth}
        \centering
        \includegraphics[width=\textwidth]{l1.jpg}
        \caption{$y=5/x$}
        \label{fig:five over x}
    \end{subfigure}
    \caption{Three simple graphs}
    \label{fig:three graphs}
\end{figure}

It didn't work and the console said:

Latex Error:
Environment subfigure undefined.
Type H<return> for immediate help.
...
l.368 \begin{subfigure}[b]{0.3\textwidth}
? H
Your command was ignored.
Type I <command> <return> to replace it with another command, or <return> to continue without it.
?

It says my command was ignored. why?

I check, the latex file and the files are correctly named and in the correct location.