It does require people to know what a traffic light is, and those colors are a convention, that's true. However, if they know what a traffic light is, and if they've used Windows before, no explanation is necessary. There are plenty of things that switchers have difficulty with (like the fact that 'minimize' and 'zoom' behave differently in OS X than they do in Windows), but this isn't one of them.

Also, even if people didn't understand the traffic light metaphor, a quick mouseover would clear it up: if you hover over the red button, it turns into a (X), if you hover over the yellow button, it turns into a (–), if you hover over the green button, it turns into a (+).