Rheology is the study of a materials flow behavior under applied deformation forces. 

HEUR (hydrophobically modified ethoxylated urethane) is an example of one type of a rheology modifier.  This associative thickener builds structure by forming temporary interactions between its hydrophobic groups and other components in the formulation such as pigments, binders, and fillers.

The associations creates a shear-thinning network allowing materials to flow under stress while maintaining structure at rest.  In paints and coatings, this translates into practical benefits like improved spatter resistance, application control, and overall film performance.