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".
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".