There is a standard analogy that helps to explain what this means. From a distance, a hose looks like a one-dimensional line, but zoom in and you see that it is in fact a two-dimensional surface wrapped around into a cylinder. The second spatial dimension (the circular direction around the hose) was too small to be seen from a distance. Klein suggested the same thing applied to Kaluza’s fifth spatial dimension, which was curled up into a circle a billionth of a trillionth of the size of an atom.
