It's on the end of the intake. You can't see it, it's located inside the front arch under where the battery is.
To access it you remove the inner arch liner (or at least drop down the front - you can leave it attached at the back of the arch, so no mudguard off etc.)
You then need to remove/plug a couple of the vacuum pipes once you remove it.
Pretty straight forward, less than an hours job.
There was a thread about a week ago that described what you need to do with the vacuum pipes, should be able to find that if you try a search.
EDIT: Didn't answer what it is

It's basically a plastic box with a flap that opens/closes at different engine loads.
It's designed to quieten the intake when the engine is under load, that's all.
Think of it like an exhaust silencer, that's on the inlet instead and is variable.