Do you mean why is it there or why did it light up? It's there to tell you the system has passed a bunch of self tests during the first seven seconds after turning on the ignition switch, then the light turns off. When it turns on at other times, the computer has detected a problem, set a diagnostic fault code in memory, and turned the system off. You need to have the code(s) read. That will indicate the circuit or system that needs further diagnosis.