A harness is a set of straps that are put on a horse so it can be hitched to a wagon or a carriage. The harness distributes the effort across larger sections of the horse's body.
Other animals and people can also wear harnesses. Your dog might have a harness for walking, with the leash attaching to the harness and not the collar, keeping the dog from choking itself when it pulls. People wear harnesses for safety, when climbing trees or bungee jumping, for example. As a verb, harness means to control or direct as if with a harness for a specific purpose, the way solar power harnesses the power of the sun.