Collision Groups are used by the Physics system. Each object in the viewport can be a member of a Collision Group (CG). To see a list of available Collision Groups, open the Physics Panel. When visualizing physics data, the checkboxes allow you to select and de-select which CGs that you would like to see at any one time.
The color swatch next to each Collision Group shows the color that will be used in the Viewport for that Collision Group. Usually this is a wireframe, but this can also be displayed as a solid color with the dropdown menu. Commonly seen types of Collision Groups include:
- HEIGHTFIELD (Heightmap)