fixed gear is the most direct of drive trains: if you apply pressure on the pedals to go forward, you will go forward. If you apply pressure to the pedals in a backwards motion - you will go backwards (considering you have the skill to maintain balance).
On a single speed, forward moves forward, but backward does not move you backwards. SS operates on a freewheel - much like a modern cassette set-up on geared bikes.

Not sure if I can make it any simpler than this