The fact that this term was popularized by Alfred Hitchcock leads some to believe it is something new. In fact, it has been around for some time. In other movies, the silent-film actress known as Pearl White was the star of regular serial cliffhangers where the characters chased after some possession. She was known to refer to the item of desire as the weenie. We can look further back still into stories such as Jason and the Argonauts or the Labors of Hercules, we see it used as well. The term Golden Fleece in place of MacGuffin was not uncommon before Hitchcock ever coined his term.
Still, MacGuffin is the most common word used, so that is the term I will be explaining. The MacGuffin is a person or thing that is sought after by the characters. It drives the action but its nature is not actually important to the story. Such people or objects are easily interchangeable. In a spy story, it doesn’t really matter if there are papers, microfilm or a grocery receipt with codes jotted on the back. With fantasy, it is likely to be some artifact to save the village. In science fiction, you are likely to be chasing down some super-tech.
If you are able to switch out what the object is or who the person is without it really affecting the story in any meaningful way, then it is a MacGuffin. Most of the time it will not be brought up again before the story ends and is likely never fully explained or understood. It exists only to get the plot moving and keep it moving and its actual nature or contents are unimportant. Handled well, it may be more valuable to the plot than if it were more detailed in our understanding of it.
Multiple MacGuffins in a single story are sometimes called plot coupons. This is a matter of the protagonist needing to collect enough of them to ‘trade in’ for a dénouement. The term was coined by Nick Lowe and more or less means a conflict resolution. If your story has the protagonist searching for a series of lost artifacts whose nature isn’t understood but that are needed before they can achieve some goal, then they are probably MacGuffins. With all of this said, don’t get too caught up in the term. You aren’t probably basing your story around the MacGuffin. It is there to use when you simply need a way to get the plot going without having to focus on an otherwise unimportant thing.