The Buff Banner is a Weapon for the Soldier in Team Fortress 2.
It is a backpack with a yellow banner inside and a bugle which you hold.
It gives you a rage meter, that when 600 damage is dealt, you can blow the bugle which disables your weapons while being blown. After being blown (which makes it make a doot doot sound), it gives you and all nearby teammates 10 seconds of minicrits (a 35% damage increase). Using the bugle drains the meter during the 10 seconds and respawning resets the meter to 0%.
Blu Scout: Guys theres a soldier and his team over there!
Blu Demoman: He has the Buff Banner
Red Soldier: *doot doot*
Blu Scout: Fuck