Silks are the uniforms that jockeys wear. Every owner manufactures his/her own silk, which has to register thereupon. By so doing, it is easier to recognise the horses and their respective jockeys right away. Silks are worn in gallop as well as in trot races but are seen as more important in gallop races as the jockey can be seen much better.
This is why silks are only shown for gallop races at RaceBets.com.