When we work with a horse we primarily use two forms of pressure to communicate, physical (the lead rope attached to the halter, the rein, the leg, the seat, etc.) or spatial (not touching the horse but being able to influence his brain and movement.) Vocal commands are a third, less common form of pressure.
A horse’s natural response to human pressure is to flee from it, become defensive towards it, or physically “challenge” it, which causes him to be unable to “hear” the person. The horse needs to learn that pressure offered by a person can be similar to feedback he'd receive from interacting with other horses.