Final answer:
Iago and Montano's conversation is about Cassio's drinking problem and how it could affect his work for Othello.
Step-by-step explanation:
Iago and Montano's conversation in Othello is about Cassio's drinking problem and how it could affect his work for Othello. In Act 2, Scene 3, Montano expresses concern about Cassio's behavior and mentions that he saw Cassio and engaging in a fight. Iago uses this opportunity to manipulate Montano and convince him that Cassio is unfit for his position as Othello's lieutenant.