Each horse in a race has to carry a certain amount of weight. To make sure that it does so, all jockeys must weigh out before a race to make sure they and their kit (including the saddle) are the right weight. If a jockey is lighter than the weight the horse has to carry, the difference will be made up by thin lead weights in a special saddle cloth.
Traditionally weighing scales with seats have been used to weigh jockeys, but these have been replaced by digital scales. You may see examples of the old type of scales in the public areas of the racecourse.
Once the jockey has weighed out, he hands the saddle to the trainer or the trainer’s assistant to saddle up the horse.
After the race the jockey must weigh in with all his kit, to confirm that the horse carried the right weight. When all the jockeys have been weighed after a race, you will hear the racecourse commentator announce “Weighed in. Weighed in.”
Trainers prefer jockeys to be as close to the allocated weight as possible, as it is harder for the horse to carry this than a human which can move with it.