What are tag groups?

Have seen this while creating and editing a category, however cannot find any UI or documentation on this.

Everything I search for ends up with group specific threads, nothing related to tag groups for categories.

The purpose of Tag Groups is

Add tags to groups to manage them more easily.

So it is more of a time saver the more tags there are.

The UI is accessible from the Tag pages, via the Admin menu at the upper right.