Recurring interviews give you a way to schedule, for example, interviews for volunteer signup:
These aren't "real" interviews in that they don't appear in the Interview Manager and can't be edited individually. Instead they're "virtual" interviews: when a volunteer searches for an interview time online, they'll be shown these interviews (in addition to any that have been created explicitly). If they sign up for one of these it becomes an actual interview in their profile and in Interview Manager.
When searching for interviews, VSys:
It then shows the remaining ones to eligible volunteers.
Changing a recurring interview (this tool) does not affect any interviews already scheduled.