Act I. Scene I. - Venice. A Street.
Roderigo to Iago: "Thou told'st me thou didst hold him in thy hate."
Iago, a soldier under Othello's command is arguing with Roderigo, a wealthy Venetian. Roderigo has paid Iago a considerable sum of money to spy on Othello for him, since he wishes to take Othello's girlfriend, Desdemona as his own.
Roderigo fears Iago has not been telling him enough and that this proves Iago's real loyalty is to Othello not him. Iago tells us of his hatred towards Othello for choosing Cassio as his new lieutenant and not him as he had expected.
To regain Roderigo's trust, Iago and Roderigo inform Brabantio, Desdemona's father of her relationship with Othello, the "Moor" which enrages Brabantio into sending parties out into the night to apprehend Othello for what must obviously be in Brabantio's eyes, an abuse of his daughter by Othello...
The play begins at night in a street of Venice. Roderigo, a prosperous Venetian Gentleman is arguing with Iago, a soldier.
He explains that Iago has had his purse (been paid by him) to keep him informed of Desdemona's activities, a women whom Roderigo is in love with (made clearer later in the text).
He complains that it angers him that Iago has not taken his duty to Roderigo seriously enough and instead has acted as if he owned Roderigo's purse (wealth) "As if the strings were thine [as if the strings were yours]," (Line 3) not telling him immediately that Desdemona had married Othello (Lines 1-5).
Iago complains that Roderigo is not listening to him. In response, Roderigo says he thought Iago hated Othello who like Desdemona is yet to be mentioned by name.
By implication, Roderigo is trying to figure out whose side Iago is...