You have probably seen how atoms are made up from a nucleus, consisting of protons and neutrons, with around that nucleus electrons. For a while, it was thought that these electrons move around fixed orbits, like planets move around the Sun. However, in the early 20th century, some physicists found out that objects that are really really tiny (like electrons) act really really weird. One way in which they act so weird, is in the fact that they don’t have a well-defined position. This means that for electrons, unlike, for example, a marble or a ball, you can’t determine the exact position $x$ at each time $t$. Instead you can only tell with a certain probability where an electron will be at a certain time. As a result, while an electron is spinning around an atom, it doesn’t do so at a fixed orbit, it will pop up at different distances from the nucleus. For example, if we would consider a hydrogen atom in the simplest possible state (the so-called
ground state”), and you would shrink yourself until you are tiny enough to sit on the proton that makes up the nucleus, you would see the electron, while spinning around the nucleus, sometimes pop up rather far away from you (this won’t happen to often, as that probability is rather low) and most of the times very close to you (there’s even a high probability it would pop up right next to you sitting on the proton!).