That's correct, it is a heat shield, and from what I've read it works quite well. However, I've also read not to use these in grass because while the area immediately under the heat shield is protected, the area around it isn't. More than one person has burned a donut in their yard.
I picked up a new fire pit two weeks ago, but haven't been able to use it. The manual that came with mine strongly encourages filling the bottom with sand to protect the porcelain. I know some people add a grate too, but that isnt mentioned in the manual.