During our regular game session, everyone keeps saying,"We need to find the mcguffin!". Is this a new sandwich or something?
A MacGuffin is a device, object, or abstract need whose finding or obtainment is the force that pushes characters and evildoers into action.
While MacGuffins is a typical plot device to push a story forward, in D&D is rather typical (I would say almost cliched) to use a broken MacGuffin, where the single pieces must be found and assembled. This allows you to carry long campaigns with different settings easily.
I strongly advise against the use of the so called Red Herrings (which are false MacGuffins that divert the attention from the actual plot/MacGuffin) in roleplaying. Players generally don't expect their attention to be diverted or the plot to be too intertwined. If they hunt a red herring and find out it is indeed a red herring, they could consider the DM wrong or accountable for having changed the story under their feet.
It's a McGuffin when it's got guff in.
"guff", of course, meaning "bluster and nonsense".
Players are being sarcastic when they say this, a bit. They're expecting that they're only going after the object of their quest because the GM said "go get it", and if and when they actually manage to find it, it will have no material impact on anything and the GM will say "great! now go get the other thing".
Alfred Hitchcock coined the term: (rough transcription, punctuation mine)
A McGuffin -- you see in most films about spies. It is a thing that the spies are after. In the days of Rudyard Kipling it would be the plans of a fort on the Khyber Pass. It would be the plans of an airplane engine, and the plans of an atom bomb, anything you like.
It's always called the thing -- that the characters on the screen worry about, but the audience don't care.
Having studied up on what makes a good thriller work in order to make good thrillers that work, he realized that you don't really need to explain certain things to the audience all that much; you only need to show that characters care about them, and if the audience sympathizes with or roots against the characters they'll believe in the characters' motivations. By the time that characters have lined up to risk their lives over this thing, the actual importance of it won't matter as much in the story as the struggles that the characters engage in over it. Is the good guy gonna win? Is the bad guy gonna get it? The Maltese Falcon in The Maltese Falcon is a classic example. The briefcase with dawn inside from Pulp Fiction is a more modern one. Maybe the most elementally pure McGuffin is in North by Northwest, where a man gets divebombed in a cornfield over nothing more definite than "government secrets".
A problem occurs, however, when you're trying to translate plots with that concept at their core over to a medium where the audience is also the characters. Can you motivate your character to care about something when you have no idea why they should? Well, perhaps you can, if you acknowledge at the same time you're acting a bit silly in the name of fun.