Daytime running lights (DRLs) are designed to make your vehicle more visible in bright, daytime conditions. They should come on automatically when you start your engine.
They go off automatically when you switch your headlights on as DRLs are too bright to be used at night when they would cause dazzle
They don't have to be separate lights and are sometimes combined with the front position lamps (side lights). If this is the case, the DRLs dim when the headlights are turned on.