The electrons around atoms are not fixed in space. In fact, we cannot not predict the exact spot in which we will find an electron if we try to measure it. We can only assign a probability to each point. If we measure the position of this electron, we are testing this probability. Let’s repeat it several times after each other and let’s take the average. We will find an average distance that is called the Bohr radius. On average, the electron is at a fixed distance from the nucleus.
(I tried it with my 15-year old sister, and she understood some parts of it 🙂 )