What is "Physics Mesh" and Do I Need Them If Not Using Mesh Colliders?

I keep getting an error stating: Failed to create Physics Mesh from source mesh....

I'm not using any mesh colliders and I've read that you can disable auto-generation/baking of Physics Mesh. But what isn't clear to me is if it's something the physics engine (or even the GPU) requires for a mesh regardless, and whether it's something separate and different from mesh colliders or not.

Is someone able to explain what Physics Mesh is?

Thanks!