There are two kinds of binocular cues: retinal disparity and convergence. Retinal disparity is a measure of the difference between what the two eyes see. Objects that are close to the eyes appear the same in each eye. Convergence is how much the eyes turn inwards to view the object.
Objects that are nearby require both eyes to turn inwards to view it. The brain automatically measures convergence and retinal disparity in order to generate an estimate of distance. Binocular cues are used to estimate distances using both eyes. Monocular cues are also used to estimate distance and use only one eye. Monocular cues include relative size, blocking other objects, relative clearness and parallel lines.