An object or goal that drives the plot forward but whose specific nature does not really matter to the story.
A MacGuffin is a plot device - an object, goal, or piece of information that the characters desperately want or need, and that motivates their actions throughout the story. The key thing about a MacGuffin is that what it actually is matters far less than the fact that everyone wants it. Swap the briefcase for a diamond or a flash drive, and the story works the same way. The term was popularized by Alfred Hitchcock, who cheerfully admitted he did not care what was in the briefcase as long as the characters did.
Understanding MacGuffins frees you from overthinking what your characters are chasing and lets you focus on why they are chasing it. Your reader cares about the tension between characters, not the specifics of a glowing crystal. If you spend three chapters explaining your MacGuffin's lore, you might be missing the point of your own story.
The titular falcon statuette drives every betrayal and murder in the story, yet its value turns out to be an illusion.
The One Ring is sometimes debated as a MacGuffin, though it is arguably more - it has its own agency and thematic weight, making it a borderline case worth studying.
Plutonium cores serve as the MacGuffin, keeping the characters racing around the globe, but the real drama is Ethan Hunt's moral choices.
If your MacGuffin needs three pages of backstory, ask yourself whether the reader actually needs all of that or whether you are stalling. Keep it simple.
A MacGuffin drives the plot but its specifics are interchangeable. A Chekhov's Gun is a specific detail whose particular nature matters to the payoff.
If readers care more about the magic sword than the person wielding it, rebalance. The MacGuffin should serve the story, not upstage it.
Write a two-page scene where two characters are competing to get hold of an ordinary household object - a jar of peanut butter, a TV remote, a set of car keys. Make the reader feel real tension and stakes without ever explaining why the object matters so much. Focus entirely on the conflict between the characters.