\documentclass{article}

\usepackage[usenames,svgnames,table]{xcolor}

\newcommand{\placeholder}[1]{\noindent%
   \fcolorbox{LightBlue}{LightCyan}{%
      \begin{minipage}{0.9\columnwidth}%
         \color{DodgerBlue}\textbf{\tiny{TODO~~}}%
         \color{MidnightBlue}#1%
      \end{minipage}%
   }%
   \par%
}
\newcommand{\placeholderinl}[1]{\noindent%
   \fcolorbox{LightBlue}{LightCyan}{\color{DodgerBlue}!}%
   ~{\color{MidnightBlue}#1}~%
   \fcolorbox{LightBlue}{LightCyan}{\color{DodgerBlue}<}%
}

\begin{document}

\placeholder{Write something useful here.}
Our system is secure because \placeholderinl{figure out why it's secure}.

\end{document}
