What is a handicap race in horse racing?

Handicap races are intended to create level playing fields by forcing superior horses to carry more weight than their competitors.
Once upon a time track handicappers would keep adding weight on a champion horse until it eventually succumbed to the law of gravity.

The great Dr. Fager, for example, including more than 130 pounds in each of the final eight races of his career, including whopping 139 pounds in his final race in 1976, the Vosburgh, which he won by six lengths over Kissin’ George. Seabiscuit carried 130-plus pounds 13 times over the course of his career.

The advent of air travel for racehorses essentially put a halt to the heavy weights, since owners and trainers could easily ship to another track if they didn’t care for the assignment in a particular race.

In the 2019 running of the prestigious Santa Anita Handicap, for instance, the high weight McKenzie carried only 123 pounds compared to his less-talented competitors, who were weighted as low as 115 pounds.