what is the "density" exactly means?

[unity100]

This statement has never made any sense to me:

As the green-ray cycle or the density of love and understanding begins to take shape the yellow-ray plane or Earth which you now enjoy in your dance will cease to be inhabited for some period of your space/time as the space/time necessary for fourth-density entities to learn their ability to shield their density from that of third is learned.

its the point where dimension difference manifests. green/3rd will be different densities, dimensions, much differently (curiously) from how 1, 2, 3d manifests.