Classic full diopters reduce the close focus distance and even allow you to take macro images without using a dedicated macro lens.
Their strength is given as reciprocal of focal length in meter (so called diopter), e.g.:
+ 1 diopter = 1 meter infinity focus plane
+ 2 diopter = 0.5 meter infinity focus plane
Diopters: +1/4, +1/2, +1, +2, +3