For those not in the know, a muscle up is bodyweight movement that combines a pull up and a dip. In other words then, you start below the pull up bar and then pull yourself upwards explosively so that you move past and over the bar and can then push yourself up above it in a pressing movement.
As the movement combines these two other exercises, it also effectively targets the muscles that both sets work. From the first portion of the movement you train the lats and the biceps, while the second half of the movement works the pecs, the triceps and the shoulders. The whole thing meanwhile is very good for the core which you’re required to keep stable throughout the movement.