\documentclass{article}
\usepackage[pdftex,active,tightpage]{preview}
\setlength\PreviewBorder{2mm}

\usepackage{amsmath,array}
\usepackage{tikz}
\usetikzlibrary{decorations.pathreplacing}

\setlength\arraycolsep{4pt} % default value: 6pt

\newcommand\tikzmark[1]{%
  \tikz[overlay,remember picture,baseline] \coordinate [anchor=base] (#1);}

\newcommand\DrawBrace[3]{%
  \draw [decorate,decoration={brace,amplitude=2pt,mirror,raise=2pt}]
    (#1) -- (#2) node [black,midway,sloped,yshift=-10pt] {\footnotesize$#3$};
}

\begin{document}
\begin{preview}
    TODO!!!
    \begin{tabular}{|l|l|}
    \hline
    Tag & Daten\\
    \hline \hline
     0  & 12345 \tikzmark{a}\\
     1  & 12345\\
     2  & 12345\\
     3  & 12345 \tikzmark{b}\\
     4  & 12345\\
     5  & 12345\\
     6  & 12345\\
     7  & 12345\\
     8  & 12345\\
     9  & 12345\\
    10  & 12345\\
    11  & 12345\\
    12  & 12345\\
    13  & 12345\\
    14  & 12345\\
    15  & 12345\\
    16  & 12345\\
    \hline
    \end{tabular}

\begin{tikzpicture}[remember picture,overlay]
\DrawBrace{b}{a}{blibla}
\end{tikzpicture}

\end{preview}
\end{document}