Cassio worries that his reputation has been ruined, but Iago offers him some advice to save his carrer. What is this advice and how will Iago use it against Cassio, Desdemona and Othello later?

Cassio laments that he has lost his reputation, which is very dear to him. Iago tries to convince him that a reputation means little; and, if he talks to Desdemona, maybe he can get her to vouch for him with Othello. This will only help Iago's plan that Desdemona and Cassio are together, which will enrage Othello. Iago then gives a soliloquy about knowing that Desdemona will speak for Cassio, and that he will be able to turn that against them both.