A MacGuffin is a plot device that writers use to grab the reader's attention and drives the plot but doesn't necessarily have a lot to do with the rest of the story. MacGuffins are used a lot in film, particularly by Hitchcock. He invented the term.
My favorite example of a MacGuffin is the $40,000 in the movie Psycho. If you think about the plot, you don't necessarily think of the money, you think of the Bates Motel or Norman's relationship with his mom ... or perhaps the shower scene or the eerie scene on the stairs ... What a great film! You have to really think about the film to even remember the $40,000. It is what brings Janet Leigh's character, Marion Crane to the Bates Motel. She stole the money from her employer and she is run away. The money is only important in that it brings her away from her normal safe world into the scary world of Norman. Upon first watching this film, the money is a distraction which you quickly forget about after the shower scene.
A more recent example is R2D2 in the original Star Wars. George Lucas refers to this androids a MacGuffin because he drives the plot of the film but it has little to do with the story. All the characters in the film are brought together by R2D2 but the droids has no importance other than that in the story.