Most muscles in the body produce movement by working in pairs, with one muscle providing a pulling, or "Agonist" motion, while the paired muscle, called the "Antagonist," counters this action by relaxing and stretching. To return to the starting position, the action of the muscles is reversed. 

For example, the simple act of moving your lower arm up and down requires the teamwork of two muscles:  your biceps and your triceps.  Standing with your arm fully extended by your side, as you flex at the elbow and curl your lower arm up, your biceps (the agonist) pull your arm  up while your triceps (the antagonist) relax and stretch.  To lower your arm back down, the action is reversed:  your triceps (now the agonist) take over pulling your arm down while your biceps (now the antagonist) relax and stretch back out. 

Since most muscles work in pairs, it's important that your exercise routine be balanced and strengthen both muscles more-or-less equally.  Developing one muscle at the expense of the other increases your chances of muscle sprains and tears as well as injuries from joint overextension and misalignment.