Diopters is a measure of the power of a lens (or mirror). It is equal to 1/focal length in meters. In EM we refer to the cm measurement to calculate diopters needed to correct refraction of the eye.
A lens with a negative diopter sign compensates for nearsightedness while a lens with a positive diopter sign compensates for farsightedness
Approximate categorizations of myopia by lens power:
0.00 - -0.75 - Probably don't need glasses
-1.00 - -2.00 - Mild myopia, no differentials needed
-2.00 - -5.00 - Moderate myopia, glasses always needed
-5.00 - -10.00 - High myopia
-10.00+ - Very high myopia. Field of view significantly reduced.