A normal horse (as opposed to a pacer, who trots laterally) trots diagonally, i.e., his near (left) fore and off (right) hind move forward at the same time, followed by his off fore and near hind. If the rider rises in the saddle at the moment the near fore is going forward, he is said to be on the left diagonal; if he rises as the off fore is going forward, he is on the right diagonal. The rider should alternate the diagonal he rises on so as to share the work load evenly; in this way the chances of one side of the horse stiffening are minimized. (You should also switch the side you rise to on a pacer.)
The horses legs move in pairs at the trot, called diagonals. The left diagonal is when the left foreleg and right hindleg move, the right diagonal is when the right foreleg and the left hindleg move. When on a circle, the rider rises as the outside foreleg moves forward.