To explain briefly:
It is a joke referring to an understanding in quantum physics called "Heisenberg's Uncertainty Principle". Scientists have concluded that we can never know *exactly* where an electron is, because by measuring its momentum we change its position, and vice versa. Thus, the best we can do is map out electron density: the places where an electron should be, and how likely it is to be there. That is, "where it probably is".
Or in simpler terms. Quantum Mechanics states that if we aren't measuring the position of an electron with something (eye, instrument, etc) its not actually there. But as soon as we are then it changes position back to where we think it should be.
Its fair to note Einstein didn't believe in this because he didn't want to believe that if we weren't looking at the moon, then it wasn't really there. Also fair to note that Einstein although smart, isnt the final say on if something is true or not.
It is a joke referring to an understanding in quantum physics called "Heisenberg's Uncertainty Principle". Scientists have concluded that we can never know *exactly* where an electron is, because by measuring its momentum we change its position, and vice versa. Thus, the best we can do is map out electron density: the places where an electron should be, and how likely it is to be there. That is, "where it probably is".