The expression "If thou canst cuckold him, thou dost thyself a pleasure, me a sport" means that if he can succeed in making Othello's wife (Desdemona) unfaithful, he will not only bring pleasure to himself (a personal gain or satisfaction) but will also provide entertainment ("sport") for Iago. This line portrays how manipulative Iago is as he incites Roderigo to pursue revenge on Othello by encouraging him to attempt to make Desdemona unfaithful.