Iago convinces Othello that Desdemona has been unfaithful.

Iago had his wife "borrow" the handkerchief from Desdemona. Iago is able to manipulate the handkerchief so that Othello comes to see it as a symbol of Desdemona herself, her devotion and finally her infidelity. The handkerchief is used as proof that she is cheating with Casio and making a cuckold out of Othello