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The lens (part right behind the pupil) in your eyeball focuses light so it hits the retina in the back. When you have good vision, the focusing is good and your brain processes the light well. When the focusing is not good, you have refractive error. The optometrist goes through and tests your eyes on various corrections to see which one works best.
An optometrist or ophthalmologist tests different aspects of lens shapes and accounts for other problems (e.g. astigmatism) to determine your prescription. A lens-maker then machines a piece of transparent material, typically some plastic, to match the prescribed shape (and may use a specific material or treat the material in some way to account for things like glare), making a lens. This is then set into a frame and that frame then altered to fit your head so that the lenses are where they should be with respect to your eyes.
Underlying all this is the way that light moves. Locally, light moves in a straight path in open space; moving through a refractive material causes the light to change direction a bit as it passes through. At the scale of light passing through an artifical lens before arriving at the natural lens of your eye, the artifical lens can be shaped in a specific fashion such that it corrects for deformities of the natural lens that distort the focus of images you ultimately see. “Correct”, here, refers to adjusting your resultant image to fit a benchmark for human eyesight (e.g. 20/20 vision refers to a clear image at 20ft, 20/100 vision means you must be within 20ft to see how a normal person would at 100, etc.).
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