what does iago tell cassio to do after being dismissed? why?

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 help Iago get the impression across that Desdemona and Cassio are together, which will make Othello very angry if it works. 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.