Is the normal distribution term D in the Cook-Torrance BRDF supposed to be between 0 and 1? What does this number actually represent?
The term D is a probability density function (PDF) of microfacets' directions. Vaguely speaking, it says what "portion" of all microfacets points in the given direction.
Since it is a PDF, it doesn't have to be less or equal to 1. It can, and often is, higher than 1 for some directions.