One of the first things you learn about driving is how traffic lights work. This is an easy concept most of us are taught as children. Green means go, yellow means slow down, and red means stop. As we get older, we also learn that there are more nuances than that.
Green means go when it is safe to do so. When the traffic light is yellow, you should slow down and stop unless you are too close to the intersection to do so safely. You should always stop at a red light, although you are allowed to make a right turn on red in some cases.