Cabosil is a resin thickener. Not to be confused with Microballoons which is a resin “extender” that mixes with resin to get super-strong glass bubbles into the medium and "fluffs it up", (not getting technical).
Cabosil, on the other hand causes the resin to have increased internal cohesive forces. In low quantities, it causes the resin not to sag or run. In higher concentrations, it causes the resin to get thicker and more like a paste. If you mix in a cup, you will inevitably entrap air and the resulting paste will turn white. If you heat the resin (<100°F) and apply a vacuum, you can get the resin to make a clear, jelly-like colloidal.