An open ending leaves the central conflict or a major character's fate unresolved when the story concludes. It doesn't wrap everything in a neat bow. I get why some people hate them—it can feel like the author just gave up or is trying to be pretentious. But when it's done well, it's masterful. The suspense comes from handing the tension over to the reader's own imagination.
Like, think about that romance where the couple reconciles but you don't know if they'll make it long-term, or a thriller where the villain's body is never found. Your brain keeps working on it for days. You fill in the blanks with your own hopes and fears. The story stops on the page, but the emotional stakes keep playing out in your head, which is a different, more personal kind of suspense than just waiting to see what happens next.