Thats a pic of an original R1100 chain tensioner, it works by oil pressure, the groove in barrel lets oil escape at a measured rate so to maintain chain tension, the whole lot is better replaced by the later hexhead chain tensioner which cures start up rattle.
The item is hollow, and if you mean what goes inside, there should be a spring.
The metal piece you're holding is only half of the cam tensioner. The other piece slides into the piece you're holding and screws into the hole in the picture.