The phrase "to become a mediator" means to take on the role or responsibility of being an impartial third party who helps two opposing sides reach a mutually agreeable solution in a dispute. Full definition
You deal with a special company which becomes a mediator in your relationship with creditors. (personalmoneyservice.com)
The hours that participants attend are necessary and are often a prerequisite to receiving credentials to become a mediator for court systems and government agencies. (wisegeek.com)
Teaching for meaning requires the teacher to become a mediator of thinking. (ascd.org)