Jared, one of the developers, describes it over at github really good - here's a quote:
"Collaborators
A Collaborator an End Users in addition to the ticket owner who is included on responses and communication in a ticket. Collaborators can be added or removed at any time in the life cycle of a ticket. Collaborators can respond in and contribute to the issue of the ticket"
Source:
https://github.com/osTicket/osTicket-1.8/wiki/Who-Are-The-Users