Humans (and most predators) have two eyes in the front of their head. This “binocular vision” improves depth perception, letting a hunter estimate the distance to its prey.
In addition to stereoscopic vision, depth perception also comes from a number of monocular depth cues (depth perception cues that can come from only one eye, or more precisely, that come from the 2D version of the picture that you see). These cues are important to good 3D video, as your brain will expect your stereoscopic perception to closely match your 2D perception of the scene you are viewing.
Monocular cues include:
Your memory of the shape and size of different objects: combined with the relative size of the image you see, this lets you perceive the distance to that object. For example, in the photo below, if you are familiar with the size of the bricks that the squirrel is standing on, you can quickly perceive the size of the squirrel, and your distance to the squirrel.
Perspective: Objects at greater distances appear smaller than near objects. Parallel lines appear to converge as distance increases. This effect is obvious as you stand on a straight road or path and look down the road, or when you look up at a tall building.
Occlusion (interposition): If we see two objects, where the first object is blocking part of a second object, we recognize that the first object is closer. In the photo below, you can tell that the tree in the center is closer than the building because it is blocking your ability to see part of the building. Occlusion helps us estimate the relative distance of objects in the photo.
Shadows and Highlights: Help us to see objects that are raised above or recessed into a surface. In the photo above, we can see that there are bumps on the tree trunk, thanks to the shadows and highlights.