The security light is a visual indication that something in the car's antitheft system is failing. In most GM cars and trucks there is a hidden system that most people are unaware even exists. From the moment you put your key in the ignition, there begins a constant line of communication between your key's security chip or transponder, to the ignition's lock cylinder, and then to the car's TDM. (Theft Deterrent Module) The TDM is what tells your computer that the correct key is being used and that it is "OK" to allow the vehicle to start and run. These 3 parts must be in CONSTANT communication. If the key is in the ignition, then these parts are sending important signals to each other. The security light illuminates when that signal gets broken, even just for a second. When the signal breaks, (normally due to a failing/broken part) the car goes into antitheft mode. The car misinterperates this and thinks its being stolen or hotwired because it is NOT getting that signal from YOUR specific key. There is where the security light turns on and shuts down your vehicle. So basically, if that light is on, your system has some sort of failing part. This can start as an intermittent "no start", to your vehicle not running for days at a time, to the vehicle NEVER starting again. The problem is electrical and computer based, so don't count on an easy reset button, or the problem fixing itself. The hard part is isolating which part is the problem. You can spend a thousand dollars through a mechanic very easily. I have had great luck with bypassing the entire system. This way the problem will never return. It actually CAN'T ever return because it is eliminated. This can also be done yourself at a much cheaper cost. Read through this link very carefully. You'll find alot of information and pictures, along with ways to help you fix this.