If you "close" your elbow bringing your forearm to rest against your upper arm, then your elbow is exhibiting:

By "closing" the elbow, we are exhibiting flexion.

The act of flexing or bending a joint, in this case the elbow, is to approximate the parts it connects to. This action is possible because of the muscles around the joint, called the flexors (Brachialis, Brachioradialis and Biceps brachii muscle)