In an event, there are scenario where you want to hide one ticket under another ticket. For example you want Ticket T2 to be active only when the person decides to buy ticket T1. in this case, you need to decide the Parent Ticket T1 for the Ticket T2. This way , a rule is defined and set for the availability of the Child Ticket.
In order to facilitate this, an option is provided in the Ticket page under setting option where you associate the parent ticket with the child ticket.