Metaphysics is a term which takes its name from the writings of Aristotle. However, it has always been an elementary and integral part of philosophy. Metaphysics deals with the basic questions of existence; what exists and what is its nature.
Socrates does not usually make strong overt claims about existence or its nature. When he does so, he tends to content himself with assertion and gives no account of his epistemology. In the Apology, for instance, he says that it is wicked to do wrong, and to disobey one's superior, but he does not say why. In this case, one might argue justification by pure logic: wrong is, by its very nature, a thing which it is wicked to do, and if you acknowledge that someone is your superior, you are simultaneously saying that he has the right to your obedience. Socrates, however, does not even make this argument.
It is also in the Apology that Socrates makes his famous statement that he is wiser than other men only in his awareness that he knows nothing. This is his usual line in approaching metaphysics: proving that others have no basis for their beliefs rather than setting out any positive vision of his own. In metaphysics, as in every other branch of knowledge, Socrates is fundamentally noncommittal and questioning.