Dramatic Irony is a very effective device used in story telling used to raise tension in a scene. It’s when the audience possess knowledge that certain characters do not. Like when you watch a horror movie and you know the killer is hiding in the closet, but the soon-to-be victim in the room has no idea. You scream at the screen telling him (or her) to get out of the house. That’s dramatic irony.
Or in JAWS. The woman swimming has no idea that a great white shark is about to attack her… but we the audience do. That’s dramatic irony.
Or the famous scene in Louis’ restaurant in THE GODFATHER. We and the Corleones know that the gun is hidden in the toilet. But Sollozzo and McClusky have no idea. Tensions rise as Michael converses with the two men until Michael finally excuses himself from the table. The whole time, we the audience waits anxiously in anticipation, wondering if Michael will go through with the plan. That’s dramatic irony.