Angle on the bow is the angle between the line of sight and the target's bow measured to port and starboard of the target's bow from 0 degrees to 180 degrees. It's difficult to describe it better, but there's an extract from the US Submarine Torpedo Fire Control Manual 1952 which has pictures:
http://www.hnsa.org/doc/attack
To me, it's the angle that the watch crew would call if they sighted my periscope from their ship - angle off their bow.