When a horse trots, it moves two pairs of limbs diagonally. So, the left front moves with the right hind, then the right front with the left hind.
These pairs are sometimes called diagonals. When rising or posting to the trot, the horse is more balanced if the rider sits down as the inside front leg goes forward and rises as it goes backward. This is called being "on the correct diagonal" and riders are often told to "watch" or "check your diagonal." (When on the trail, riders posting to the trot change diagonal regularly so as not to put extra stress on one pair of legs over the other).