In the human body, the motion of bones is made possible by muscles that are attached to the bones with tendons. Muscles contract when motion is required, they cannot elongate from their relaxed length. To make any bone move a set of muscles attached in opposite directions is required.
The main muscle that contracts when any bone has to be moved is called the primer mover (another term for the same is agonist). The motion of the bone in the opposite direction is made possible by the other muscle also known as antagonist. For example, the biceps are the agonist in the motion about the elbow and the triceps are the antagonist. Synergists are muscles that aid the agonist by properly defining the direction of motion. They help to cancel excess force exerted by the agonist that could lead to motion in a direction it is not required. An example of a synergist are the brachioradialis and brachialis which assist the biceps when motion about the elbow is required.
For the most part muscles work in pairs or groups. The muscle that is working is called the prime mover. The other part of the muscle pairing or group is the synergist. The prime mover is helped by the synergist. Synergists are sometimes called neutralizers because they help to cancel extra motion from the prime mover. With these two parts working together it allows muscles to work efficiently and within the same joint motion.