A trope is a common theme used in storytelling. For example, a kiss awakens a sleeping beauty or transforms a froggy character into a prince. The effect of the kiss varies but it’s the same trope.
A MacGuffin is one of my favorite tropes. A MacGuffin is a plot device—often in the form of a physical prop—that is used to drive character motivation. The all-time classic example is the bird statue in The Maltese Falcon, but we see roughly the same thing with the ark in Raiders of the Lost Ark. Characters are desperate to find the MacGuffin, evildoers would do anything to possess it and the plot goes nowhere without it.
Some say a MacGuffin is a writer's crutch, but the device has been employed so many times in highly successful books and films that I think of MacGuffins as more like lost car keys; you’re not going to get very far without them. Sometimes the MacGuffin turns out to be unimportant or never found, such as the grail in Monty Python and the Holy Grail, but a MacGuffin can become a crucial plot element, such as Dorothy’s ruby slippers in The Wizard of Oz, their unknown power only revealed at the end.
Another of my favorite tropes is Phlebotinum, which is defined as “a versatile substance that can be rubbed onto anything to achieve an effect needed by the plot”. If your character must be hidden, give him an invisibility cloak. If your spacecraft is on a long voyage to the stars, you’d better have artificial gravity. No need to explain how it works—it just does. Once you understand this trope, you see it everywhere. James Bond always has exactly the right function on his super-fancy wristwatch to get out of a jam. The Nazi general in Wonder Woman ingests capsules that give him temporary super strength. Why? Because otherwise Wonder Woman would just lasso him and the story is over.
Phlebotinum is unexplained science or magic that has both power and limitations. The rules of Phlebotinum are often arbitrary once you think about it, but they always serve the plot. The genie in Aladdin can grant three wishes with a restriction that he can’t make people fall in love. Handy for the climax.
You might think of these plot devices as corny or overused, but in the hands of a good writer you might not even notice them. 2001 – A Space Odyssey is unquestionably one of the best science fiction films of all time, yet the mysterious monolith is both a MacGuffin and alien Phlebotinum. Its presence on the Earth, moon and Jupiter drives character motivation and its powers (teaching apes and humans or screeching when touched by astronauts) are precisely what is required to take the story to its conclusion.