Symbolism, Imagery, Allegory
The most dominant symbol in the play is the handkerchief that circulates throughout the play. That's right: in one of the world's most famous tragedies, the #1 symbol is not something super-dramatic like a throne or a sword, but a measly (pardon our French) snot-rag.
Because Othello gave it to Desdemona as a first gift, the handkerchief functions as a token of his love, which Desdemona cherishes:
I am glad I have found this napkin.
This was her first remembrance from the Moor.
My wayward husband hath a hundred times
Wooed me to steal it. But she so loves the token
(For he conjured her she should ever keep it)
That she reserves it evermore about her
To kiss and talk to. I'll have the work ta'en out
And give 't Iago. What he will do with it
Heaven knows, not I.
I nothing but to please his fantasy. (3.3.334-343)
This is why Iago convinces his wife to steal it from Desdemona—he knows that it has a lot of sentimental value and that Othello will be angry when he finds out his wife no longer has it.
Iago also knows that, for Othello, the handkerchief symbolizes Desdemona's fidelity. When it shows up in Cassio's possession, Othello is convinced that Desdemona is unfaithful. The white napkin, as we know, is spotted with red strawberries, and Othello tells Desdemona that the strawberries were hand stitched with thread that has been dyed with blood from "maidens' hearts" or, virgins' blood (3.4.87). Which: eew.
In this way, the handkerchief resembles a white wedding sheet that's also been stained with a virgin's blood. (Again: eew.) So, in Othello's mind, as long as Desdemona has the handkerchief in her possession, she's chaste. But the moment she "loses it," she loses her chastity.
The handkerchief also seems to function as a symbol of Othello's mysterious past and his "exoticness." He tells Desdemona that an Egyptian "charmer" gave it to his mother and that it would keep his father under her spell (3.4.67). That such a small object has such enormous weight in the play testifies to the sensitivity of jealous minds, and the way that small incidents can be magnified psychologically into "proofs" of love or betrayal.