It means that in some languages, some pronouns take into consideration many more different criterions than in others.
For example, in English, there are only 6 prounouns with 1 to 2 layers each.
She: gender = feminine, singular.
They : gender = feminine or masculine, plural.
In French, we have two different "they", one for females groups (elles), one for male groups (ils). So we are superimposing more layers.
Hope that makes sense :)