What does the sphere of my prescription say about my visual acuity? What is sphere?

Near or far sighted. Sphere is the degree of near (minus power) or far (plus power) sightedness. For example, -3.00 sphere is 3 diopters of nearsightedness. -4.00 would be even more nearsightedness. Put a + in front of those numbers instead, and that would be farsightedness. Sphere does not include any astigmatism, a.K.A. Cylinder.