Do you mean a maintainence light or a Check Engine/Service Engine Soon Light? If the Check Engine light/Service Engine Soon light illuminates while you are driving, the car's on board computer sees a fault in the engines management system that is causing an excess amount of pollutants to be emitted from your car. This could result in engine damage if left neglected. Some problems that can cause the Check Engine light to stay on may take two "drive cycles' to complete before illuminating the Check Engine Light. The computer sees a problem and looks for an identical set of circumstances to occur to confirm that it has indeed seen a problem before turning on the light. A Check Engine light that continually flashes while you are driving is serious and is a sign that the computer sees a problem that seriously risks damaging the catalytic converter which may result in an expensive repair or even may cause a fire! Diagnostic codes are stored in the vehicle's computer and can be retrieved using a scantool that communicates with the computer in your car. We have in our Repairpal website an encyclopedia with in depth articles about computer, and computer control systems, emission and driveability problems. http://repairpal.com/check-engine-light. Codes need to be retrieved, investigated, repaired and then turned out using a scantool.
My Tahoe manual does not indicate that you CAN reset the "check engine" light. You can reset the "check engine oil" light after having a oil change. I did notice the "check engine" light come on once as I was backing out of my garage. Apparently, I had pressed the button located at the end of the column shift lever, (which is used to assist when towing a heavy load). I drove like this for about 3 miles at my normal speed and noticed the engine felt really sluggish. I pulled out my manual to see what this "check engine" light was indicating, and then noticed the "trailer tow" light had started flashing. I never even realized my Tahoe had this feature before accidently engaging it while shifting into reverse. It was as simple as pressing the button at the very end of the Shift lever again to take it out of tow mode. The manual indicates an engine problem if the light comes on and stays on. Possibly due to something as easy as a recent fuel change with lesser grade fuel or the fuel cap not on tight. Possibly something more complicated.