Muscle tissue is made of cells whose cytoplasm can contract. When the muscles contracts, it become short, and thereby pulls on the bone in which it is inserted. When you want to show someone how strong you are and you "make a muscle" by contracting your biceps, your forearm is pulled up toward your shoulder. If you want to lower your arm, you relax your biceps and contract your triceps, the muscle on the underside of the arm.
The contraction of the triceps pulls the forearm straight. You can see that the two muscles of the upper arm work as a team or pair. All the voluntary muscles of the body work in pairs.
