"Diopter" is just the optical power of a lens, a unit of measure. Like a close-up lens for your camera... with your camera lens set to infinity focus, a one-diopter closeup lens causes it to be focused to one meter. A two-diopter closeup lens results in focus to 1/2 meter, etc.
Correction lenses can also be attached to your camera eyepiece to help your eye focus on the viewfinder's focusing aid, whether it's the RF spot or the SLR ground-glass. These also are made available in various strengths depending on your need, in diopter units. Thus the lenses are often called diopter lenses; may not be the correct term optically, but it's a common usage anyway! Some cameras have built-in adjustable eyepieces, and the adjustment feature may be called the diopter correction.
As rover alluded to, with the viewfinder corrected for your eyesight either through an add-on "diopter" lens or the body's built-in correction feature, you then can probably use the camera without your eyeglasses. I think most cameras standardize on -1 diopter for the normal correction, and you go from there.
