The Requester of a Ticket is the person (or persons) that are waiting for the resolution of the issue.
Normally, every ticket has one requester. There is, however, no such restriction in RT. In fact, it is common to have more than one requester when two or more tickets are merged. It is even possible that there is no requester on a ticket: it may be that the original requester is no longer interested in the resolution of the issue, but it will be treated anyway.