The above quote originates from Act 1 Scene 3 in Othello where Roderigo complains bitterly about losing Desdemona to Othello. Iago, of course, is unsympathetic. He advises Roderigo to pull himself together and to keep his wits about him.
When Roderigo whines that he might as well die, Iago pipes up:
Oh, villainous! I have looked upon the world for four times seven years, and since I could distinguish betwixt a benefit and an injury I never found man that knew how to love himself. Ere I would say I would drown myself for the love of a guinea hen, I would change my humanity with a baboon.
Here, Iago says that, in all the twenty-eight years he has lived on earth, he's never met a wise man who knew how to promote his own interests effectively. Then, he brings in the imagery of the 'guinea hen' and the 'baboon.' The guinea hen is an euphemism for a prostitute in the Shakespearean era. Basically, Iago is saying that he wouldn't even think of drowning himself for a woman of such ill worth as Desdemona. It's all pretty insulting, but he's trying to get Roderigo to calm down because he needs his help to destroy Othello.
The baboon reference is an insult to Othello; it is a reference to the animal sexuality of a black man in Shakespearean England. Remember that in Act 1, Iago warns Brabantio that 'an old black ram (Othello) /Is tupping your white ewe (Desdemona).' Again, the black ram reference is a sexual one, just like the baboon reference; the two imply that all black men are hyper-sexual creatures who will ravish innocent and defenseless white women. Iago appropriates the imagery of animals to refer to Othello, the man he thoroughly despises; he's saying that he would never exchange his humanity for that of a baboon's (of course, here, Othello is the baboon).
If you want to know more about Shakespearean insults, try reading this book:
Shakespeare's Insults: A Pragmatic Dictionary.