$parallax is a material shader parameter available on the
PBR (Strata shader) shader in Strata Source. It enables Parallax mapping on the surface of the PBR material.
This parameter also works with Thexa4's PBR shader.
Parallax mapping is a shading technique used to give perceived depth to brushes with this applied. In order to have this work correctly, you will need to have a Heightmap embedded in the alpha channel of your Normal map. This can be done with a program like GIMP, and a tutorial on how to do this can be found here
Defines the height of the highest point of the texture. Essentially how close to the face the texture appears. This is necessary to adjust correctly, otherwise the material may appear to stretch unrealistically past the end of the brush. "0.5" is a good starting value.
Defines the lowest point of the texture relative to the height. Essentially the intensity of the heightmap. "0.1" is a good starting value.