I've just assumed it had something to do with the intake. But what exactly is it and what does it do?

 

it doesnt do anything you just try to get rid of it and hawk up a big loogey. Ooops did you say plenum or phlegm? I always get those 2 mixed up ah nevermind.

 

A plenum is the chamber through which air passes as it is routed to each of the 6 cylinders. In some cars the plenum also serves as the chamber in which the fuel and air mix further on their way to the cylinders.