To me, it means that the only thing that anyone can know is their own mind. In other words,, if a person thinks that they are correct, they dont really know that they are correct; all they really know is that they think they are correct. Even if a person feels one hundred percent certain,, abundant evidence indicates that they are correct, no evidence indicates that they are incorrect, and everyone agrees: even then, they could be mistaken because people do to know the world, or the universe; they know their own minds.
It's okay though. Knowing only our own minds has allowed us to function, and survive, as well as we have.