A length is a period of elapsed time on the horse racing track. But actually, the term indicates the difference between the winning horses. It determines by how far one horse is from another- and the commentators usually declared by how many lengths the winner has beaten the runner up. It is usually the length of a horse- which measures 8-9 ft from the tip of its nose to its tail. The first length is always the length of the winning horse. So, when the result is that the winner beat the runner up "by two lengths", it means that another horse would fit in in the distance that remained between the tip of the tail of the winner and the nose of the runner up.