It's a math formula. The prescription tells the lens maker how to shape the lens (glasses, contacts) that will best correct your vision. The first number is the amount of nearsightedness (-) or farsightedness (+) to be corrected. The next number is the amount of astigmatism to treat and the number after the "x" is the angle of the astigmatism to be treated.
Optical correction. The prescription for glasses describes the optical correction necessary to help you achieve the best vision. The first number describes the spherical correction (+or-)and the next 2 numbers detail the cylindrical correction (+ or -) and the angle of the correction necessary to overcome any astigmatism (ovalness0 detected by the refraction exam of the eye.