What do you think "density" means?

[Plenum]

In fact, it seems that the lower the density is, the more concentrated or dense the matter is, and the higher the density, the more concentrated the presence of mind or awareness (and the lighter the manifestation of matter).

yeah, I think that's right.  Whenever Ra referred to greater 'denseness' it was always in relation to light.  So the physical vehicles get progressively more 'light packed' and 'dense'.  Density is definitely in the context of 'light material'.

Personally I think they mean "density of consciousness".  I think that a greater amount of "spiritual gravity" (which is synonymous with polarity) results in a higher concentration of awareness. It would seem to me that higher densities are NOT more dense with matter.  

I personally think that the physical vehicle associated with each density directly limits the experiential consciousness possible.  So basically the mind complex puts a brake on 'pure consciousness' and runs it through the 7 rays associated with that particular density.  Ra mentioned (in one of the most delightful answers) that the rays in each density have such a different meaning, that it's hard for us to conceive.  And so the amount and quality of work is vastly different, even though the schema of the 7 energy centres holds throughout the densities.

This is why dual activated individuals have such a leg up on 3d-physical natives.  They have a relatively unimpaired connection to their spirit complex; as opposed to everyone else who has to develop it by experience in that lifetime.  That's their birthright - and also the gift to all positive graduates who are making Earth their new home.  The rest of us can only wonder as to how and why they are so clear and articulated in their 'understandings of love'.