There are two common ways that manufacturers use to describe the densities of an ND filter.
One is with a ND number, such as ND2, ND4, ND8 etc. and this refers simply to the ratio of light it will allow to pass. A ND 4 allows 1/4 of the light to pass. This is common with photography filters.
The second is by optical density, this will be 0.3, 0.6, 0.9, 1.2 etc. which is most common for ND filters used in cinematography.