The line T-Rex is talking about is in Act I, Scene III of Hamlet. Polonius is giving advice to his son Laertes, and it is basically about how to be a worthless tool who doesn't have an opinion of his own. This is because Polonius is a worthless tool who doesn't have an opinion of his own. He is "that guy," the one who doesn't commit to anything, and will support whatever seems popular. This is why it is ironic when he says, "This above all, to thine own self be true, and it must follow as the night the day thou canst not then be false to any man."
Many people interpret this line to mean something like, "You gotta keep it real!" Or, "Do what you want with your life!" Which is worthless advice, as T-Rex points out: "Some people's true selves are total chumps!"
There is another interpretation to the line, though, that might have some wisdom in it: if you are honest with yourself, then you will be honest in your dealings with others. It is very easy for us to lie to ourselves, and delude ourselves into believing things that we only want to be true. It seems like we only hurt ourselves when we do this, but we also have the potential to hurt others. We might promise to do things that we might not actually be able to do, or misrepresent what our goals and intentions are. If we are first honest with ourselves, and then equally honest with those around us, people can have more certainty in what to expect from us. For instance, they can expect us to talk about ourselves in the plural.