Close up lenses (sometimes called 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.

This classification is the same system used in eye glass and contact lens rating.