The Merchant of Venice opens with Antonio, a merchant, feeling very sad. It would seem he is concerned, or so his friends believe, for his ships. Apparently, they are at sea and in jeopardy and could be wrecked in dangerous waters. Antonio's wealth rests with his ships and the possibility of shipwreck and the significance of Antonio's melancholy mood, foreshadow what is to come in the personal lives of Antonio and his nemesis, Shylock.
Antonio disagrees with Salerio and Salanio's assessment of his feelings but is aware that something is wrong. He does not know "what stuff tis made of"(I.i.4) or "whereof it is born." The "it" refers to his feelings of sadness and he is therefore saying that he does not know the origin of his feelings (his sadness). He does not know where the feelings come from.