I'm not sure what the plus and minus mean. I see here on the prescription form that it's labeled "sphere," so maybe it has something to do with how your lens deviates from being perfectly spherical? But I don't know, and (this will come as a shock to some) I don't feel like looking it up right now.
Oh, who am I kidding.
There are three main parts to a vision correction prescription: the sphere, the cylinder, and the axis. The sphere represents the strength of the lens measured in diopters, which is the reciprocal of the focal distance of the lens measured in meters. My sphere is +1.25 in my right eye, and +4.0 in my left. So if two parallel rays of light pass through the right lens of my glasses, the rays will converge at a distance of 0.8 meters away; the same two rays passing through my left lens will come together a mere 0.25 meters away. A lens with a positive sphere will magnify an image; a negative sphere will make the image smaller, by dispersing the light that passes through it. A spherical lens works in all directions equally.
The cylinder and the axis indicate the degree to which you are astigmatic. Correcting for astigmatism requires a cylindrical lens rotated to a specific position. The cylinder is measured in diopters, the axis in degrees. A cylindrical lens corrects in one direction only; the image will be corrected in height or width, but not both. The axis of rotation determines what will be magnified and in what direction.
I got this information from RudyRX.com and Specs2Go.com.