Your lenses suck at focusing, that's why you need glasses. Focusing just means bending the light to go to a smaller point at a particular distance. Do you know what light doesn't need to be focused? Light that's already on its way to that point. So what happens when you squint? You're actually removing all the light that requires bending so what's left is the light that would already hit your retina correctly. You're effectively turning your eye into a pinhole camera