This is a Section Waitlist feature for admin-managed Sections.
Admins can add a waitlist to each individual Section. Once a Section's waitlist has been enabled, students who want to join a Section that is at full capacity have the option to join its waitlist instead. This feature allows students to see all offered Sections within a community, regardless of their current capacity.
The full feature is described in our Help Center article, Sections Waitlist.