The phrase "to light up" means to turn on or activate something, often referring to a source of illumination. This can be done by switching on a switch, pushing a button, flicking a toggle, etc., in order to make an object emit light. The term is commonly used when talking about turning on lights such as lamps, candles, or electric bulbs. It could also refer to activating something that emits light for the purpose of illumination like a torch, flashlight, or even fireworks.