Roderigo has reached the end of his rope, again, and is telling Iago that he must either have results for all his payment or that he is going to quit the whole affair. Once he threatens to tell Desdemona of all his doings and this is a great threat to Iago's plans.
So Iago lays on the praise nice and thick, tells Roderigo that now he sees the strength and passion in him and he says that he believes that things will really work out this time. Iago tells Roderigo that he will be with Desdemona by the next night or Roderigo can torture him to death.
Iago tells Roderigo that Cassio and Desdemona are going to run off together. So Iago sets him up to duel with Cassio, promising Roderigo that he will be able to kill him, leaving Desdemona for Roderigo.