Density is the measurement of how tightly or loosely a given substance is packed into a given volume. Air, for example, is low density, much lower than human tissue, which is why we can pass through it. The same does not apply to granite. Don’t try to walk through granite. It’s bad for you.
Density is one of the basic properties of physical science. Physics, chemistry, and even biology require a clear understanding of what density is and how it’s calculated.