I imagine that a 15-year old has already seen rudimentary atomic models with electrons moving around a nucleus.
Imagine that you are the nucleus and an electron is orbiting around you, going in circles on its path. Take a piece of paper and note where that electron is at every 10 seconds. At first, the average of your notes will fluctuate quite a bit. After all, the electron will one time be further away and another closer. But if you take notes long enough, you notice that you will get an average position. This is the time-averaged position of that electron..