A tune up is basically checking existing parts and replacing any that need it. It's done to make sure that the engine is running in the best condition possible, and is considered preventative maintenance. Here's a general list of things:
http://www.ehow.com/how_1874_tune-car.html
It's not usually urgent, unless your car is running bad. But it's certainly something that should be done in the relatively near future, since it's an 11 year old car.