Medal of Bravery is a PvP reward from participating in either ground or space based PvP scenarios while you are the rank of Lieutenant Commander. You received Medals of Bravery from winning or losing a scenario, and the number you receive appears to be based on your overall performance. You can expect to receive 18-25 Medals from each scenario you take place in. Along with Medals of Bravery, you will also receive Energy, Starfleet Merits, Bridge Officer Points, and Skill Points for your effort (good or bad) in a PvP scenario.