The side of the pitch where the ball is. When defending, you want more players ball side than away from it, since that's where the danger is. "Getting ball side" means positioning yourself between your opponent and the ball. Defenders who stay ball side cut off passing lanes; those who get caught wrong side get played in behind and face danger from fast attackers.
The first thing coaches teach young defenders is to stay ball side. If you're marking a striker and the ball is on the right, you need to be between that striker and the ball, not standing goal side waiting.
Running toward the far post from a cross, arriving late into the danger area. The cross travels across the face of goal, past the near post runners and the goalkeeper, and finds someone arriving at the back. Defenders struggle to track back post runners because their eyes are on the ball and not behind them. It requires timing and pace to arrive at the right moment with a back post run, but it is a very effective tactic.