The Buff Banner is secondary slot item that has can only be used once you fill up your rage meter, done by damaging your enemies.
When full you can activate it by sounding the bugle which will deploy the flag on your backpack, boosting your fellow teammates. The buff works in a sizable area around you, giving all friendlies involved mini-crit damage for 10 seconds. This includes yourself, and even local sentry guns.
The Buff Banner is awarded for achieving Milestone 3 (Complete 17 Soldier achievements)