Cassio has been walking outside the walls since he lost his place. He goes to Desdemona, as Iago told him to. The request is that she turn the general’s mind back.
Desdemona takes it on at once. She raises it with her husband again and again. She even asks him to name a day.
From a distance Iago watches that scene with Othello. He says nothing at all. He only makes a small sound as Cassio leaves.