Tuning has to do with setting up your bike to fit your preferred riding style and comfort. This can be done by, as YZ125 said, removing the governors for added speed, adjusting the suspension to your liking, and bolting on or modifying a myriad of performance parts to suit your individual needs.
Commonly upgraded parts are carbs/intakes/air filters, exhaust, rocket key, clutch, and sprockets/pinions. There is a huge How-To section on this forum with topics on just about anything you might have the interest to do. If you have specific questions you should post threads on the Midbike forum, as that is technically what your x18 is. Bear it mind that just about any question you ask will have already been answered in another thread, so don't be surprised if someone just directs you to that.