With the standard configuration, one might not be able to insert a footnote with the plain command:
\footnote{...}
The result is that we can see the footnote icon, but the content is nowhere in the whole article. This is because the footnote command is sheltered by the tabular environment.
To achieve the possibility of inserting footnote in tables, we can actually cover the tabular environment with another handy environment, which is provided in the "footnote" package:
\usepackage{footnote}
\begin{savenotes}
\begin{table}
\begin{tabular}{|c|c|}
...
\end{tabular}
\end{table}
\end{savenotes}
Then within the table, the footnote command can be used freely like in other parts of the text.
Reference:
http://texblog.org/2012/02/03/using-footnote-in-a-table/
