Putting several footnotes below the table

You should use the threeparttable package, that's designed for that sort of things. In the following code, I added the caption package, to ensure a proper spacing between caption and table, bookttabs and the makecell package, that allows for common formatting of column heads.

\documentclass[12pt]{article}
\usepackage[utf8]{inputenc}
\usepackage{booktabs, caption, makecell}
\renewcommand\theadfont{\bfseries}
\usepackage{threeparttable}

\begin{document}

\begin{table}[! htbp]\centering \caption{Summary Statistics}
\begin{threeparttable}
\begin{tabular}{l c c c}
\toprule\midrule
\thead{Variable} & \thead{Mean}
 & \thead{Std. Dev.} & \thead{N}\\ \midrule
a & a & a\tnote{*} & a \\
\bottomrule\addlinespace[1ex]
\end{tabular}
\begin{tablenotes}\footnotesize
\item[*] Blahblah
\end{tablenotes}
\end{threeparttable}
\label{table2}
\end{table}

\end{document}