One of the most commonly used plot devices out there is the MacGuffin. Popularized by Alfred Hitchcock, the term typically describes an object that is important to the characters in some way and helps move the plot forward. In Hitchcock's original description of the term, he also mentions that the MacGuffin doesn't actually have to be important aside from to the characters at that specific point of the story. Once it has served its purpose, it often becomes far less useful to the characters.