This is a wooden box belonging to Booker DeWitt. The box is handed to him by a couple in the rowboat at the beginning of the game.
On the top it says:
"Property of Booker DeWitt. 7th Cavalry, Wounded Knee (Note: This is NOT a Skyrim Reference or Easter Egg. It references the Wounded Knee Massacre in South Dakota, an actual historical event)
Big metallic key