This is my understanding of the difference. I'm sure someone on the forum will correct me if I'm wrong.
Normal train wheels are basically square if looked at from the side.
Fast angle wheels are angled.
What this does is reduce the friction between and wheel and the rail because less area is touching and it allows the wheel to slide easier from side to side on the track on curves or areas where the distance between rails might vary slightly. Less friction means less drag on the engine and couplers.