The serratus anterior are located just under the arm on the ribcage and are embedded within the lats. Developing the serratus can be a huge asset for bodybuilders seeking greater upper body width.
But unless you're a boxer, MMA fighter, or some kind of athlete that does explosive pressing, throwing, or punching movements, most training neglects the use of this muscle group.
The serratus elevates and protracts the scapulae, and nearly every upper body movement in the gym asks for the scapulae to be depressed and retracted ("shoulders set" position) in order for the movement to be performed correctly.
Most people wouldn't think there are many ways to specifically target this muscle group, but try this:
Serratus Front Raise
Set up an incline bench in front of a cable machine. Set the pulleys at the bottom and sit down in the bench, facing the machine. Begin a front raise movement, except don't focus on the deltoids being the prime movers. Keep your elbows slightly bent and let the serratus pull your shoulders apart and upwards.
Don't go too high or your delts will take over. Level with the face should be the correct landmark for the finish position.