1) what is the range of possibilities for what it will do
2) an associated probability of occurrence for each of those possibilities
Both of these things are modeled mathematically in what is called the wave function. We have one wave function and a host of different operations we do on it to find 1) and 2). The catch is that when we measure the atom, only one possibility will manifest. The wave function will change accordingly to which possibility manifests. We then have a NEW wave function and a new set of possibilities and probabilities.
The strange part is that the atom behaves as if it is in every possible state that the wave function represents - until it is measured when one pops up on the measuring instrument. There is no way to say how or why an atom behaved in a particular way. The reason why physicists believe this is complicated. If you are interested look up "bell's theorem."
The many worlds theory says that the other possibilities happen too, just not to us. Each possibility happens, but in a different universe. The reason why physicists consider it as a possibility is because the math works. There is no evidence to support it yet though.
What quantum mechanics does say beyond a doubt is that the universe is spontaneous - and it is all one. What happens to one atom can effect another - instantly - all the way across the universe.