A traffic light is an object, usually found hanging in the middle of intersections of roads, and usually not found on highways, except in intersections in small towns. The purpose of the traffic light is to control the flow of traffic at that intersection so everyone can drive through safely. Most traffic lights have 3 lights on them, Red, Green, and Yellow. When the light is green, one can safely pass through, if no one is running a red light. Yellow means caution, because the light is about to turn Red. Red means stop, don't go. While you "have the red light" another road "has the green" and they can go.
Some traffic lights have more than those three lights. These extra lights are usually green, with an arrow shaped gobo infront of the light. These lights are used to tell when it's safe for a lane to turn, or not. If the arrow is green, it is safe. If the extra light is not green and the traffic light displays green for your road then turn when it is safe too.