What is Mesh Density Info ?
Mesh density info is a small tool which allows you to know the density of vertices by unit scale of each objects you've selected. That means that it gives you the information of density regarding to the place your object takes in your scene.
Why it can be useful ?
When you are dealing with grounds, for animation or scattering, you need to have a good vision of the density of it, but don't want it to be over subdivided.