It’s pretty simple. The cornea and the lens needs to focus light right onto the retina in the back of the eye in order for us to see clearly. If our corneas and lenses are screwed up, it’ll pinpoint the light in front of or behind the retina causing us to only be able to see what’s right in front of us or what’s really far away. Basically, slapping a pair of glasses on your eyes refocuses light so that it hits the retina, as it’s supposed to. The prescription is adjusted to what you need.