Although the eyes are almost fully developed at birth, the brain is unable to make sense of the information being sent by the eyes, so vision is very poor over the first few months of life.   For vision to develop normally, the eyes must be straight and be receiving a clear image. 

If for example, one eye looks slightly inwards, the image will not be clear.   The part of the brain receiving information from this eye will not develop properly and the eye will become lazy (amblyopic). 

If this is picked up early enough, the good eye can be covered with a patch. This will get the lazy eye working again. In most cases the vision in the lazy eyecan be greatly improved with this treatment.