what is the "density" exactly means?

[zenmaster]

What is this means that human is the entity of third density or animal is the entity of second density? Is this means that the mind or the body is in the specific density?

For the 2nd density animal, the mind and body are in 2nd density.
For the 3rd density human, the mind and body are in 3rd density.

The bodies of both beings are in space/time, while the minds are in time/space.

Each higher density subsumes the lower's qualities.

Does a fourth density entity have a body like us?

I know that they can and do have bodies like us. This body is apparently difficult, but not impossible, to manifest in our 3rd density. For 5th density, it is apparently simple.

Or they don't need one because they live in the time/space dimension.

They are still centered in space/time, within their dimension. However, 4th-density time/space is much more transparent to them than our 3rd-density time/space is to us.