It's an emissions thing. It pulls air from your airbox, through a pump, and into your exhaust system on top of the engine so that unburned gasses will combust in your exhaust, most commonly heard as the backfiring when closed throttle, slowing down etc.
In relation to tuning, it gives false air fuel readings as it's injecting air into the exhaust side of the system, so blocking it off allows a tuner to properly do their job.
There's numerous ways to remove the system, the right way is with a cap on the airbox nipple and block off plates on the engine block, the lazy way is to just plug the holes with whatever your crafty self can come up with like marbles and screws and clamps or in this case, some gorilla glue.
Shut up and ride.
Nun sacciu, nun vidi, nun ceru e si ceru durmiv