\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{equation}
  D_{it} =
    \begin{cases}
      1 & \text{if bank $i$ issues ABs at time $t$}\\
      2 & \text{if bank $i$ issues CBs at time $t$}\\
      0 & \text{otherwise}
    \end{cases}       
\end{equation}
\end{document}