Compound exercises as the name suggests work multiple muscle groups at the same time. For example, a squat is a compound exercise that works the quadriceps, glutes, and calves.
Compound exercises are prescribed, as they replicate movements that are more natural and ensure the individual is progressing towards complete wellness as efficiently as possible.
Take the squat again; it is replicating the movement for standing up, driving forward, jumping and many other movements you might see in real life or sport, with each muscle getting attention during the movement.