The phrase states that "when you are nothing" you are irrelevant to everyone around you. In this case, there is no pressure for you to reach someone's perspectives, or assume a certain behavior, belief and religion, for example, to please people. Thus, you can make free decisions, since nobody cares or expects anything from you.
When that happens "you're free to believe anything", that is, you can take your own thoughts without external influences, you can assume your own standards and value what you think is correct, without any judgment.