How do glasses work?
Glasses are comprised of lenses that bend light. When a beam of light passes through any curved piece of glass, it has a tendency to either expand the beam and spread it out, or make the beam narrower, eventually into nothing, then expand again.
How the eye works is that natural lenses are able to bend the light that passes through it into a clear, right-side-up, image at a precise point in your eye that makes the image clear. Lenses in your eye have the ability to make an image right-side-up, but sometimes lenses don't make your vision clear, or make it too clear.
A lens in the eye may bend the wrong way and completely miss its target for clear eyesight. Glasses bends light further to adjust the light that finally goes into your eye, eventually correcting the light that goes where it should.
The thicker the glasses, the greater the bend, the same can be said on how curved the glasses are. There are many factors that determine where light goes in your eye that extensive precision is taken into account when making prescription glasses.