As said before, Binocular Vision (BV) is essentially how well your eyes work as a pair. This is useful for us in a variety of ways. Having two eyes, and the position they take on the front of our head is the result of millions of years of evolution. Our eyes both face in front of us, which means there is a fair amount of overlap in what each eye sees. Light will pass through our pupils and hit the retina at the back. The light that comes from straight in front of you will hit what is called the fovea. This is the most sensitive part of the retina, and creates the best quality image. You can demonstrate this difference by focussing your eyes on one word here, and trying to read the words two lines above. Your brain will struggle to make out what it says! When you are looking at something, and the light entering both eyes is hitting that fovea in both eyes, this is called bifoveal fixation. This leads to the best vision you can have.
Unfortunately, this isn't always the case. There are a few different ways in which the eyes do not focus at the exact same spot. It could be that one of your muscles that control your eyes isn't working as effectively as the rest of them. There are 6 muscles which control where your eyes look. These are called the extra-ocular muscles (EOMs). They work as pairs, pulling against each other to keep your eye looking where you want it to. However, if one of those muscles isn't as strong as its counterpart, it won't be able to hold your eye where you want it to look. In this case, the eye may be pulled off to one side, creating what's called a Strabismus, or a "squint". These muscles are controlled by one of the 12 cranial nerves. The muscle weakness can sometimes come from damage to this nerve. This can occur as a result of trauma, or from an incorrect formation at birth.
Another reason strabismus can occur is due to Amblyopia, the vision in one eye being poor. If one eye can't see well enough to fixate properly, then it may sit off at a different position to the other one. This usually occurs during childhood, and is sometimes known as a "lazy eye". Often, children will not notice there is something wrong as their other eye is compensating for this poorer vision, and this highlights why it is important for children to get regular eye tests. If this is not found by around the age of 6, it can be very difficult to correct this issue.
If you have a strabismus, your brain may not be able to match up the images each eye sends to it, resulting in diplopia (double vision). Not only is this very distracting and annoying, it can prevent you from knowing where something actually is. Our brain can sometimes compensate for this, by either training the eye which is not fixating properly to allow the images to be seen as one, or by shutting down the vision this eye sends to the brain.