In the movies, an object which is critical to the characters, and the plot of the movie, but is of no direct relevance to the audience, is known as a MacGuffin.
A MacGuffin can be any object, as long as it is desired by a character or characters, and drives the plot forward.Popular MacGuffins in movies include treasure maps, holy relics (a chalice, for example), suitcases of money, stolen paintings, a roll of film, valuable gems, or a secret weapon.
A true MacGuffin must not be 'used' as such, and should not actually be able to do anything, except act as an object of desire to a film's protagonist/s and/or antagonist/s.
Famous examples of movie MacGuffins are the Maltese falcon in The Maltese Falcon, Rosebud in Citizen Kane, The Holy Grail in The Da Vinci Code and Rose's necklace in Titanic.
A MacGuffin can also be a place, or even a time.
This is a wonderful animated YouTube short has audio of Alfred Hitchcock describing a MacGuffin to interviewer Dick Cavett: