What do the middle fibers of deltoid muscles do?

Deltoid muscle fibers can be divided into anterior, middle, and posterior fibers (portions). Although the functions are named the same as the deltoid muscle, it's important to note that, while the anterior fibers function to flex (bring forward) the arm, the middle fibers abduct (move away) the arm and the posterior fibers extend (move backward) the arm.

So specifically the Middle deltoid fibers' function is to abduct the arm (i.e., move the arm away from the midline of the body) and assist with arm flexion (moving the arm forward and upward).