PxrBump used to provide the secondary bump on displaced tiles.
The modern and preferred solution for bump mapping is now the PxrBumpMixer node. This node is maintained for backwards compatibility and might be removed in a future release.
Bump mapping is an inexpensive way to apply details to an object without modeling it or using displacement to change its shape. This is a shading "trick" where a supplied pattern can give the illusion of details.
This produces a bumped normal from a bump map (a scalar displacement map, so only the first channel is used for the displacement). Bumps are applied in object space. The change in scale between object space and current space is automatically applied by the node.