The cathode is the electrode where the reduction occurs.
The reduction is the gain of electrons.
In any voltaic cell, the half-reaction with the highest (most positive) reduction potential will be the one occurring at the cathode (reduction reaction).
So, Br2 + 2 e - → 2Br -, Eo = + 1.07 V is the half reaction that most likely occur at the cathode of an electrochemical cell.