Med-arb usually refers to a process where mediation and arbitration are combined. For example, the parties may start arbitrating a dispute and then agree to attempt mediation. Or they may commence mediation and then when they are unable to reach agreement, agree to arbitrate (i.e. arb-med).
What is unusual about this is that the same person may act as both mediator and arbitrator. In China this practice is relatively common, however, in western jurisdictions this practice is often viewed with concern. There is concern that it is difficult, if not impossible, for someone to act as both mediator and then arbitrator. As a mediator you may well learn confidential information, which you may have to ignore when sitting as a decision maker.
The practice of arb-med has now been specifically recognised in Hong Kong by its inclusion in the Arbitration Ordinance (Cap. 609).