In sum, the argument goes like this: if there is more than one universe, then there will be some universes that are larger than ours, some that are smaller, some that are younger and some that are much, much older. If we, in our universe, have evolved intelligence sufficient to construct machines to simulate simple mammalian brains, then it is reasonable to assume that beings in a much larger, much older universe would have computational power far in excess of ours. So much so, that not only would they be able to simulate entire smaller, younger universes, but those simulations may themselves have the ability to simulate even smaller, younger universes. Turtles all the way down, so to speak.
So even if the very biggest, oldest universes form the long tail of all possible universes, the fact that they could simulate universes that simulate universes that simulate universes means that, all else being equal, the probability that our universe is real is much less than the probability that we are, in fact, in a simulation.