A striplight lighting fixture is a long row of small connected lights, used to have multiple colors of light coming from roughly the same direction. Usually theaters use gels to make these lights different colors. The unit can then be wired into several different circuits, allowing each bank of colored lights to be controlled by a separate dimmer on the lighting board.
Striplights are being slowly replaced in newer theaters by cyclorama units, which can produce similar effects with a motorized reflector.