The security system on todays cars and trucks prevents the vehicle from being stolen. There are many variations of the system going back to the early 1990's.
This warning could be on when the engine is running, indicating a fault has been detected in the system. If left un repaired, there is a good possibility that the car will eventually not start.
There are several different possibilities for the cause of the warning, depending on the make / model / year of the vehicle. Anything from a bad ignition cylinder wire, to a bad cylinder to a dad Body Control Module (BCM).
After market (non-factory) alarm and remote start systems can cause havoc with the factory theft system, causing this warning light, or a no start.