A roll back in jumping is when there are 2 fences next to each other and you jump one turn and jump the other one. Sometimes they are set on. Bottom line: Mastering the rollback will improve your horse's steering, He should rock his weight back on his hindquarters and jump his. Either turn and immediately jump back to your start position, Immediately return to seated position, and roll back down, keeping knees pulled.
When that is really smooth turning both directions, and when you have a soft, waiting horse on landing, THEN do jump, rollback, jump. Use your speed and momentum to roll back and forth, one vertebra at a time, Practice doing squats and frog jumps first, and then advance to rolling squats.