Questions about encouraging activity on CGCC, and/or attracting/retaining users.
The activity level of CGCC depends on that of its users; more users means more questions, more answers, and more moderation capability. Sometimes, people will flag up specific problems that might cause activity to drop (e.g. "site policy X is offputting to new users"), or suggest solutions that might help encourage activity; this tag is used to categorize the resulting discussions.
It could also conceptually be used for questions about what levels of site activity are desirable (without the automatic assumption that it's a good thing); however, at the time of writing this tag wiki there have been no such questions asked so far.
Note that site-health was more of an issue prior to CGCC's graduation into an official non-beta Stack Exchange site (as the health of the site was a large factor in whether it would graduate or eventually be abandoned), and thus many of the related questions date from the time of CGCC Beta, discussing how the site could be made more likely to graduate. It's less commonly used nowadays, but occasional discussions about the future of the site are still definitely likely to be helpful!