The term diopters refers to a measurement of how much a corrective lens has to bend light to focus it on your retina in order to stabilize your vision. This is the +/- measurements you see on your eye-glass prescription. If a lens can bend parallel light rays to a focal point of one meter it is said to have a power of 1 dioptre or 1.00D. A 2 diopter lens can focus light at a point of .5 meters away from itself. Only a curved surface is able to bend light.
If you have a near-sighted or myopic correction of -1D it means that you are focused at 1 meter which is about the length of your arm. This means that if you take off your glasses, objects further away than your hands begin to appear blurry, and less visible the further away they become. If you are -2D than your focus is at .5 meters away, or half the length of your arm and so on. So the higher the diopter correction, the stronger the correction is.