what is the "density" exactly means?

[unity100]

The alleviation mechanism was designed by the placement of this consciousness in second-dimensional physical chemical complexes which are
not able to be dextrous or manipulative to the extent which is appropriate
to the working of the third-density distortions of the mind complex.

this doesnt seem to imply dimensionality like that.

it basically says, the advanced spirits were put into less capable bodies.