Amblyopia is a problem with the brain as a result of other problems with the eye. Commonly referred to as a "lazy eye" by practitioners (which can be confusing because lay people often use that term to describe a wandering eye, aka strabismus), amblyopia typically results in one eye that sees worse than the other eye, despite wearing the appropriate correction and having no other medical conditions. Two particular factors that can cause amblyopia are a large difference in prescription between the two eyes, and strabismus. When the brain receives two images that it cannot put together to create the 3D picture that we normally see (because one is clear and one is too blurry or because they don't align properly), the brain is forced to ignore one of those images in order to make sense of the data. As the brain develops, the processing area that should have been split 50/50 between the two eyes is instead predominantly used for the preferred eye. Later, when the blurred vision and/or misalignment is corrected, the brain is accustomed to ignored the non-preferred eye and has less processing power to devote to it.
If amblyopia is discovered at a young age - typically under 10 years old - the brain can reconfigure itself and restore binocular vision with some training and therapy. This is much less likely to happen in later years. Many amblyopes will have one normal eye with good vision and never complain about any problems. This is one of the many reasons annual exams are recommended for all school-age children, even those who don't complain about their vision.