Follow-Up:
It depends on why it came on. If it's flashing, you need to take it in for service ASAP. If it's just steady on, it can mean anything from a loose (or missing) gas cap to a couple of misfires to something very wrong with the emissions systems (like the catalytic converter or various oxygen sensors). You were told correct in that it will take a mechanic to tell you why it came on, or at least someone who has VW diagnostics equipment (VAG-1551 or VAG-COM), but depending on why it came on will also depend on if it resets itself after so many miles, hours, or after the condition is corrected. For example, a couple of misfires could result in it automatically shutting off the light after a while, but a failed oxygen sensor will keep it on until the sensor is replaced. How is the vehicle performing otherwise? That's usually a good indication of whether or not you have a serious issue. The vehicle does have a built-in "limp home mode" which will result in you not being able to get it higher than second gear and the engine will not allow itself to be rev'ed over a certain RPM or it will stall. If everything else seems fine, check your gas cap and if you feel like risking it, wait a few miles/days to see if it goes off. Not knowing what's going on was what caused me to buy VAG-COM ($200) -- might be worth it to you, too.
Here is a link that might be useful: MotorHomepage Rialta Resource