Our muscles have peripheral nerves running throughout them, and when we move around to do things in our daily lives, we rely on muscle contraction to help us perform these physical activities. Although this action is usually conscious, sometimes muscles can contract on their own. This happens when a motor nerve fires on its own, causing the muscle fibers that it's connected to to start twitching.
These twitches are called fasciculations (fah-sick-you-lay-shuns).