Amblyopia is a deficiency of visual acuity  in an otherwise normal eye. In relation to squint, the lack of a clear and focused retinal image in the squinting eye leads to suppression of input from that eye by the visual cortex. The brain cannot make sense of 2 different retinal inputs, and so ignores one of them. In children whose visual systems are still forming,  amblyopiaa is irreversible after the age of 7, when binocular visual reflexes mature. Therefore it is extremely important to align eyes properly to prevent amblyopia becoming permanent in later life.