It's a reciprocal angle to the target. Roughly, the bearing a lookout on the target would give if he noticed your periscope. So port zero-niner-zero would indicate that the target was dead ahead, and you are on the port (left) side. Zero angle means he's heading directly for you, one-eight-zero directly away. Combined with range, target speed, own-boat speed, and target bearing, it will allow you (or the TDC) to caculate the correct gyro angles for the torpedoes.

J.T. McDaniel