The Shear modulus, sometimes called the rigidity modulus, refers to the change produced by a tangential stress, and provides a measure of how "stiff" the material is.
You do not need a fiber-filled material to produce this microcellular result, because the bubbles themselves will affect the mechanical properties of the material.
Shear modulus is a mechanical property value. The distribution of this mechanical property is used by the structural analysis for its performance evaluation in a Stress analysis.
This Shear modulus (Microcellular) results is a laminate-based result. You can check the shear modulus in each laminate by animating the default contour plot, which will animate the result over the normalized thickness. You can check these result for more detailed information on the shear modulus distribution.