Brabantio is a Venetian senator and Desdemona's father. He is quite protective of her and has kept her from many suitors including Roderigo, who he has forbidden to ever visit his home again. Desdemona has fallen in love with Othello and has eloped with him.
Iago and Roderigo come to rouse Brabantio in the dead of night to inform him that Othello has abducted his daughter against her will and is abusing her at the very moment that they are talking to him. Their purpose is to demonize Othello so that Brabantio may use his authority to have Othello dismissed from his post as general and have him incarcerated.
The reason for this vindictive and pernicious lie is that firstly, Iago is jealous of Othello's position since he has been appointed general of the Venetian army even though he is a foreigner and a Moor. Secondly, Iago has been loyal to Othello and has applied to be his lieutenant but Othello chose to appoint a Florentine, Michael Cassio, in the position. Iago resents this appointment since he believes that Michael has only book knowledge and lacks experience in battle, whilst he, Iago, possesses the right qualities for the post.
In his attempts to obtain the position, Iago had also acquired the recommendations of three senators but these were ignored by Othello. Iago had promised that he would continue feigning loyalty to the general so that he may serve his turn upon him.
Roderigo, who is besotted with Desdemona, is assisting Iago in his devious scheme in the hope of obtaining some advantage in his desire to woo her. Iago has promised that he would ensure Roderigo's success in this endeavor. The gullible Roderigo has now become putty in Iago's manipulative paws and pleases the malevolent schemer's every whim.
On their way to Brabantio's home, Iago instructs Roderigo:
Call up her father,
Rouse him: make after him, poison his delight,
Proclaim him in the streets; incense her kinsmen,
And, though he in a fertile climate dwell,
Plague him with flies: though that his joy be joy,
Yet throw such changes of vexation on't,
As it may lose some colour.
He obviously wishes to spoil both Brabantio's sleep and mar Othello's joy. He uses Roderigo to do his despicable work. Once Brabantio has been awoken, they will give him the frightful news about his daughter's kidnap. After Brabantio has heard their truly gross account of Othello's so-called crime, he is extremely upset, and passionately cries out:
Strike on the tinder, ho!
Give me a taper! call up all my people!
This accident is not unlike my dream:
Belief of it oppresses me already.
Light, I say! light!
As soon as Iago knows that Brabantio is convinced and has taken action, he takes his leave, unashamedly telling Roderigo that he should be seen as being in support of Othello in this matter so that suspicion does not fall upon him. Furthermore, he would be able to gain Othello's trust, thus giving him the opportunity to manipulate the unsuspecting general even more. Roderigo remains behind to provide Brabantio with more detail about Othello's supposed crime and where he can be found.
The two plotters are successful in their attempt to raise Brabantio's ire but fail to have the Moor punished since Desdemona later stands up for him and he is absolved of all guilt. Othello is then requested to leave for Cyprus where he has to fight off a possible invasion by the Turks.