Iago has persuaded Roderigo to join him on the journey, as they approach the balcony of Brabantio (Desdemona's Father), explaining how his daughter has married a moor.
Iago pouring in all kinds of alcohol in an attempt to get Cassio drunk. Iago knows that Cassio can easily get drunk, so that one glass from the bowl will get him drunk. Iago tells Cassio to celebrate after the Turkish fleet sunk by having a drink. After some time, Cassio DOES have a drink.
Cassio ends up getting drunk and angry. Iago sends in Roderigo to cause a disaster, which ends up with Cassio stabbing Montano. Because of Cassio stabbing Montano, Othello demotes Cassio from his position, and puts Iago in charge.
This is a VERY important scene within Othello. In this scene, Iago plants the idea of Desdemona having an affair with Cassio on Othello. This initiates Othello's jealousy, which is a fatal flaw he has. With the idea planted in Othello's mind, and events that will happen later in the story, it will contribute to a twisted ending full of tragedy.
This is ANOTHER VERY important scene within Othello. In this scene, Desdemona drops the prized handkerchief of Othello. Othello received the handkerchief from his mother on her deathbed as she told him to give it to the lady he loved. The handkerchief symbolically represents Othello's love and when Desdemona drops it in this scene, it sets up proof that Desdemona had an affair with Cassio on Othello.
Shortly after Cassio had lost his job after stabbing Montano, Iago explained to Cassio that he should talk to Desdemona and convince her to try and convince Othello to give him his position back. Cassio does and after some exchange, Desdemona agrees that she will talk about Cassio nonstop to Othello until he gets the job back. This is NOT great because it only suggests more proof to the affair that was planted in Othello's head and feeds to it. Slowly, Othello begins to believe it is true, especially because Desdemona won't stop talking about how amazing Cassio is.