Primarily a step light is a light fixture that can easily be fitted on any step or staircase. Shining the light downwards this then enables you to see clearly when using them during the night.
They usually have a choice of two settings, a dusk to dawn auto, which means as soon as the daylight hours start to disappear they will automatically turn on. Others will have a sensor that picks up any movement and then turn on, good for if you are arriving home late, this setting also ensures longer lasting battery life.