Stereoscopic vision refers to the ability that humans have to see the same scene with both eyes in slightly different ways. It results in our ability to visually perceive depth and distances. Stereoscopic vision is not synonymous with depth perception, but rather leads to it.
In humans and in animals with this type of vision, each eye captures a slightly different image. This difference is known as binocular disparity, or retinal disparity. The brain processes these two images in a way that lets us see slightly around solid objects without needing to move our heads. It does this by essentially pairing up the similarities in the two images and then factoring the differences into our perception of a scene. These differences are usually small, but can translate into a significantly different final result.
The visual advantages that humans have as a result of stereoscopic vision are most obvious when compared to someone who doesn't have this ability, because he has lost the use of an eye, for instance. These individuals can make certain adjustments to account for the loss of depth perception, but it is largely impossible to regain all of what has been lost, regardless of these adaptations. Stereoscopic vision is also related to our ability to manipulate small objects with our hands. Similarly, some woodland animals use it to precisely navigate through branches and other forest environments where accurate depth perception is a matter of survival.
Indeed, there is speculation that our stereoscopic vision also evolved as a means of survival, allowing us to see and assess potential threats with greater accuracy and faster response time. In our own time, many routine daily activities are facilitated by this aspect of our vision. For example, a surgeon must have stereoscopic vision in order to accurately perform a procedure, and the driver of a car must be able to tell how far away his car is from other objects. Even such a routine task as climbing a flight of stairs would be significantly impaired without this type of vision.
As helpful as stereoscopic vision is, it is not the only way we have to judge distance. Our brains can also use what is known as the focal distance of an object to estimate how far away it is. In doing this, the brain judges distance based on how the lens of the eye must change in order to bring a given object into clear focus. This gives a general idea, but is not as precise as stereoscopic vision.