(written from a Production point of view)
A concept in storytelling, a MacGuffin is a motivator in a plot, often presented with minimal or no explanation in the narrative.
Although Star Trek: The Next Generation science consultant David Krieger devised a pseudo-scientific explanation for what Krieger waves actually were (for the episode "A Matter of Perspective"), Krieger waves became a MacGuffin, in his own words, when the explanation he intended ultimately ended up on the cutting room floor. 
An article about this storytelling concept, in Star Trek Magazine issue 185 (pp. 48-53), referred to the following items as MacGuffins:
- V'ger (Star Trek: The Motion Picture)
- Genesis torpedo; Ceti eel (Star Trek II: The Wrath of Khan)
- Genesis Planet; Spock (Star Trek III: The Search for Spock)
- Whales; time travel (Star Trek IV: The Voyage Home)
- Center of the universe; God (Star Trek V: The Final Frontier)
- Planet goes kaboom; gravity boots; a ship that can fire while cloaked (Star Trek VI: The Undiscovered Country)
- The Nexus (Star Trek Generations)
- Time travel; the Borg (Star Trek: First Contact)
- Fountain of youth (Star Trek: Insurrection)
- Picard clone; radiation weapon (Star Trek Nemesis)
In the reference book Trek Navigator: The Ultimate Guide to the Entire Trek Saga (p. 32), Edward Gross referred to the concept of the Caretaker as a MacGuffin in VOY: "Caretaker".
While writing the novel Home Is the Hunter, author Dana Kramer-Rolls limited herself to including no MacGuffins to return the crew of the USS Enterprise to their own time period. (Voyages of Imagination)