They define metaphysical statements as those which are not verifiable and not a tautology. An example that is given is a real quote which I remember as something along the lines of (paraphrased) "lines of Being unfold into evolution but do not enter into it itself." Since a statement is verifiable when the speaker/listener know what sense-content is predicted by it, the point is that metaphysical statements such as these carry no factual content. Statements may still carry emotional meaning for the speaker or listener but they can't say anything about the world unless they relate to sensations in some way, since everything we perceive is through one type of sensation or another.