Car cuts off

Why does it do that

Lack of fuel or air or spark. Too many reasons. Does the engine lug before it dies (like it's under a load)? That might indicate a problem with transmission or torque converter. Does it sputter out like it's lacking fuel? Since this happens at a stop light, and assuming your foot is on the brakes and not the accelerator pedal, it may be the 'intake air control' (IAC) valve - that allows an engine to breath when you are not pressing down the gas pedal. There's about a thousand other reasons your car could die at a stop light, but at least that's a starting point. Give it a tune-up and clean all the filters and throttle body, check for vacuum leaks, scan for trouble codes, then proceed from there.