A "plot device" is a technique or method that moves the plot forward. The primary plot device found in Emilia's part in Othello is the handkerchief.
The handkerchief becomes the center of Iago's plot against Othello. As such, it ultimately leads to the death of three characters in the play: Desdemona, Othello, and Emilia herself.
Iago plots to undermine Othello by making him jealous of Desdemona. There's just one problem: Desdemona is a model wife. She's completely faithful to Othello, so there's no way to accuse her of any cheating she's actually done.
So Iago has to make something up. To do this, he recruits his wife Emilia, who is also Desdemona's lady in waiting. Torn between her loyalty to Desdemona and her loyalty to Iago, Emilia wavers at first. Ultimately she meets her husband's demands by picking up a handkerchief Desdemona drops.
Emilia gives the handkerchief to Iago, who plants it in Cassio's quarters. When the handkerchief is discovered, Othello recognizes it as Desdemona's and suspects her of cheating on him. In a jealous rage, he kills her.
Desdemona's death, however, drives Emilia to confess the entire plot to Othello:
O thou dull Moor! that handkerchief thou speak'st of
I found by fortune and did give my husband;
For often, with a solemn earnestness,
More than indeed belong'd to such a trifle,
He begg'd of me to steal it.
Iago tries to stop her, but Emilia continues: "She give it Cassio! No, alas! I found it, / And I did give 't my husband."
Furious at her for revealing his scheme, Iago stabs Emilia. Othello, overcome by grief that he has killed his innocent wife, kills himself.
The handkerchief here serves as the device by which the entire plot unfolds, culminating in the deaths of Desdemona, Emilia and Othello. Without it, there is no reason for Othello to mistrust Desdemona, and nothing for Emilia to confess.