This functionality is useful if you'd like to define a connection between multiple resources.
In order to enable the feature, you must first go to Settings/Resource Types and click on Meeting Room (or whatever name you've entered for the default meeting room resource).
After you click on it in the newly opened window you'll be able to enable the option called Allow parent/child relationship.
This can be used in a use case where if you'd like to make a booking in one meeting room it can be reflected in the other and vice versa.
By setting the same parent for two meeting rooms you can ensure that if someone books the parent meeting room, all related meeting rooms will be booked for the same time as well.
If a resource has multiple parents and you book either the child or one of the parent meeting rooms, all children and parent meeting rooms will be booked simultaneously.
This way you can create a complex network of meeting rooms that are related to each other and if one gets booked then all related meeting rooms will be booked as well.
However, if you create a booking in the past for a meeting room and then another for the same slot in its parent you'll be able to occupy the same booking slot twice.
This only applies for bookings either in the past or 1 or more years in the future, duplicate bookings cannot be created for the current year.