what is the "density" exactly means?

[unity100]

if things progress normally, i very much think there will come a time in which the increasingly 4d compatible generations will chance up right on top of a queer point at which they will be neither 3d heavy, nor 4d heavy, but in the middle body wise.

this could lead to a lot of curious phenomenon regarding paranormal stuff. ie, like going invisible a moment, then being visible (for 3d) the next, and so on.