Fusion, or binocular vision, is the phenomenon of the two eyes fixing on the same object at the same time. This requires the eyes to work together as a team. In order for the eyes to fuse on an object up close they must converge, or turn inward. When the eyes fuse on an object far away they diverge, or turn back out. Strain or tension present in one or more of the eye muscles may prevent the eyes from working together to simultaneously focus on the same point.
An excellent way to experience and practice fusion is by using a rope or a string. Tie one end of the rope to a doorknob and hold the other end in front of your nose. Begin tracking the eyes up and down the rope from nose to doorknob. If you struggle you may cover one eye at a time to get used to the feeling. When focusing up close the rope should make the shape of the letter V. When focusing anywhere along the middle the rope should make the shape of the letter X. When focusing on the doorknob the rope should make the shape of the letter A, or an upside down V.