Depth perception is the ability to see the world in 3 dimensions and by doing so to be able to estimate the distance between objects.
Retinal disparity is defined as the way that your left eye and your right eye view slightly different images.
Convergence refers to the eyes' disposition to rotate inward toward each other in a coordinated manner in order to focus effectively on nearby objects.
Disparity and convergence are the two important things that help us to judge distance. When looking at a close-up object, your eyes angle inwards towards each other (you become slightly cross-eyed). The extra effort used by the muscles on the outside of each eye gives a clue to the brain about how far away the object is. This is convergence.