Imagine that you are sitting on a chair at the center of the nucleus and you observe a ball (the electron) gradually moving from position to position. You can imagine that the floor has circular rings indicating the distance between the ball and yourself. Now, every 5 minutes, your phone will ring and when that happens, you look at the ball and write down its position. You repeat this many times and at the end, you sum all the numbers and devide by the number of times you heard the ring. This is the time-averaged position of the electron.