Close up diopters change the infinity focus of a lens to a distance much closer. They are effectively an additional lens element that is screwed onto the end of the lens – very much like a filter. They need to be the same diameter as the end of the lens.
The power of a diopter is given as a number which is the reciprocal of its focal point in metres. Thus a +3 diopter focuses at 1/3m and a +2 diopter focuses at 1/2m and so on. Diopters can be stacked together so that a +2 and +3 together produce a +5 and focus at 1/5m.