I have a 2001 GMC Envoy. The service engine soon ome on and Im going out of town tomorrow. Is it safe to keep driving? Im driving about 3 hours and then 3 hours to get back. What does SES mean? What causes it to come on?

Stupid reason: You left your fuel cap off.

It can cause all sorts of driveability problems and poor gas miliage. Jut had a MIL lamp come on today. The car was bucking occaisionally. A dirty sensor.

Unfortunately, it could be nearly ANYTHING.

Autozone scans for free and it takes a few minutes. Only then can you make an assessment.

Agree with KISS, get a scan done to rule out anything major. If it's something minimal like an emissions code, go ahead with your trip.

Generally speaking, if you're not experiencing any noticeable drivability symptoms, i.e.: bucking, skipping, surging, etc, the light is on for an evaporative emissions fault.
That means that performance won't be lost but just a tiny bit of gas mileage.

Remember, the check engine light doesn't care about how your car runs, just how much it pollutes.