Breakout rooms can be set to close manually or based on a timer.
When closing it manually, click the X in the breakout room title bar. If attendees are in the breakout room when you close it, all attendees will be returned to the main room.
To close based upon a timer, the timed option has to be set when creating the breakout room.
The minimum time is 5 minutes from when the first attendee is moved into the room.
When the set time has been reached, the room is automatically closed and all attendees are moved back to the main room.