The War Drums are a craftable musical instrument. They are placed like crafting stations but can be picked up again.
Performing a song requires placing a music blueprint in the inventory of the drums. The drummer then gets on the drums and left clicks to start the musical minigame. Successful completion of the song creates a buff to friendly players in the area and consumes the music blueprint. You can also free-play the war drums by right clicking and you will see an interface with every note you can play.