A stronger source of light. In other words, if there is a window off stage right and the play calls for daylight to be coming through the window, the lighting designer would aim key lights, representing daylight, from the direction of the window, accounting for time of day with the angle and color and other such details. The daylight is the key light. Another example would be a lamp. Since a regular 100 watt bulb is bright and distracting on the stage, a lighting designer may use multiple instruments to create the illusion of the lamp giving off light, while the lamp itself operates at a lower intensity. Also see the fill light definition for the compliment to key light.
