this is cool! what is a MacGuffin used for in movies?

A MacGuffin (sometimes McGuffin) is a plot device that motivates the characters or advances the story, but the details of which are of little or no importance otherwise.
The element that distinguishes a MacGuffin from other types of plot devices is that it is not important what the object specifically is. Anything that serves as a motivation will do. The MacGuffin might even be ambiguous. Its importance is accepted by the story’s characters, but it does not actually have any effect on the story. It can be generic or left open to interpretation.
The MacGuffin is common in films, especially thrillers. Commonly, though not always, the MacGuffin is the central focus of the film in the first act, and later declines in importance as the struggles and motivations of characters play out. Sometimes the MacGuffin is all but forgotten by the end of the film.
Examples in film:

• The top secret plans in The 39 Steps (1935).
• The eponymous statuette in The Maltese Falcon (1941).
• The letters of transit in Casablanca (1942).
• The “government secrets” in North by Northwest (1959).
• The stamps in Charade.
• The case with glowing contents in Kiss Me Deadly.
• The Ark Of The Covenant in first Indiana Jones film, Raiders of the Lost Ark (1981).
• The mysterious, dangerous contents in the trunk of the 1964 Chevrolet Malibu in Repo Man (1984).
• The unknown contents of the briefcase in Pulp Fiction (1994).
• The suitcase in Ronin (1998).
• The Process in David Mamet’s The Spanish Prisoner (1998).
• The Green Destiny Sword in Crouching Tiger, Hidden Dragon (2000).
• The “Rabbit’s Foot” sealed canister in Mission: Impossible III (2006).
• The chest in Pirates of the Caribbean: Dead Man’s Chest (2006).
• Fertility in Children of Men (2006).
• The Allspark in Transformers (2007).