Hes making a literally joke because with quantum entanglement. You have 2 particles together. One is spin up and one is spin down. But neither know which one it is until you observe one. The moment you observe one of the particles and find out its spin.. then automatically the other one becomes the opposite instantaneously. Doesn't matter if the particles are light years apart. So by using that information with the joke. He is saying as if the twins were in a quantum entanglement, once you got the information to one of them. The other would know it immediately. Unfortunately the other twin would know the exact opposite of what the first sibling knew