Iago is telling Roderigo that he has a plan to get back at Othello, for not appointing him a lieutenant. He will play 2 faced and act very loyal to Othello but plot against him behind Othello's back.
I am not what I am
Iago and Roderigo come to awake Brabantio at his house. Iago tells him Othello is having sex with his daughter right now, and that gets Brabantio fired up. Brabantio finds out this is true when he checks on his daughter's empty room and calls his men so they can find Othello
I am one, sir, that comes to tell you your daughter and the Moor are now making the beast with 2 backs
Brabantio is telling Roderigo on the way to confront Othello about how he wished his daughter was with Roderigo instead of Othello just because Roderigo is white and Othello is black.
O, would you had had her
Brabantio goes up to Othello to confront him about how he kidnapped, drugged, and raped his daughter. Othello stays very calm and wants to know where he can answer these accusations against him. Brabantio says he should go to the Duke but Othello says he already called him for military reasons.
O, thou thief, where hast thou stowed my daughter?
Whither will you that I go to answer this your charge?
Desdemona, Brabantio's daughter, was called into the "trial" and is telling the Duke that she loves him. She says that she sees how Othello sees himself, which is very important in a good relationship. This does not make her father happy and embarrasses him.
I saw Othello's visage in his mind
The Duke is telling Brabantio that Othello will be a great suit for his daughter. He states Othello is more "fair" in the way of being equal or honest more than it does by being light smooth-skinned. The Duke thinks Othello is a good man for Brabantio's daughter and calls Othello Brabantio's son-in-law.
If virtue no delighted beauty lack, Your son-in-law is far more fair than black