Clipping a horse means shaving all or part of their coat off. It is mainly done to horses working over winter who would otherwise get too hot working in a full winter coat. The type of clip (how much of the coat is clipped off) will depend on how hard the horse will be working and how thick their winter coat gets. It will reduce the time it takes to cool the horse down and make grooming easier.
Clipping is also often part of showing. It displays neatness and improves their appearance at a horse show. This is is done in summer as well as winter.