A damper is a valve or plate
that stops or regulates air flow inside the chimney.
We bought a damper thinking it was part of what we needed, but found
out just in time that our Jotul
F 602 does not require
one due to it being air tight and having a rain cap to prevent Santa and other critters from entering.
The downside to a damper is how it restricts air flow even when it's
open and makes cleaning the chimney a lot more difficult.