What does the check engine light on a 2004 VW golf mean?

What does the check engine light on a 2004 VW golf mean?
As the vehicle is running the computer is constantly doing self tests & component tests. If any component is found to be out of the specified range it turns on the light to let you know. A common problem is a loose gas cap. If you don't find anything simple you need to get the car scanned and codes pulled from the computer for guidance as to where the problem lies.