Final answer:
Othello overhears a conversation manipulated by Iago, making him believe Cassio is speaking about an affair with Desdemona, igniting Othello's jealousy and leading to tragic consequences.
Step-by-step explanation:
The conversation between Iago and Cassio overheard by Othello in Shakespeare's play is a crucial turning point in the plot. Othello is led to believe that Cassio is speaking about an affair with Desdemona, Othello's wife, when in fact Iago has orchestrated the conversation to be about another woman, Bianca. This manipulation is part of Iago's deceitful scheme to incite Othello's jealousy and lead to his downfall. The tragic outcome of this misunderstanding showcases Shakespeare's expertise in portraying complex human emotions and the catastrophic consequences of deception.