Wikipedia sums up Wittgenstein's ideas on "A limit to understanding" quite well:
"An aim of the Tractatus is to reveal the relationship between language and the world: what can be said about it, and what can only be shown. Wittgenstein argues that language has an underlying logical structure, a structure that provides the limits of what can be said meaningfully, and therefore the limits of what can be thought. The limits of language, for Wittgenstein, are the limits of philosophy. Much of philosophy involves attempts to say the unsayable: "what we can say at all can be said clearly", he argues. Anything beyond that—religion, ethics, aesthetics, the mystical—cannot be discussed. They are not in themselves nonsensical, but any statement about them must be. He wrote in the preface: "The book will, therefore, draw a limit to thinking, or rather—not to thinking, but to the expression of thoughts; for, in order to draw a limit to thinking we should have to be able to think both sides of this limit (we should therefore have to be able to think what cannot be thought).""
I disagree with him somewhat. Although many do not like evolutionary psychology, because of its implications, I think it really shines a light on how and why we evolved a moral faculty. I do, however, agree that the underlying logical structure of language puts a limit on what can be thought and understood. I would consider getting his book, but I heard it is almost impossible to understand.