Hi Chris,
Slip angle is the angle between the boat centerline and the path of the boat through the water. It is simply a weather vane that runs in the water and the base is clamped to the boat. The vane is connected to a pointer just above the clamp/bracket that indicates the direction of the vane in the water as the boat sails. The difference in the vane indicated direction and boat centerline is the slip angle.
I have done alot of sailing with instruments on my boats because I don't have another boat to sail against so I sail against the instruments. This has helped me develope boats that are the fastest US Sailing has on record and cost 1/15th as much as a Formula 40 boat for example.
Bill