Sometimes, the specific nature of the MacGuffin is not important to the plot such that anything that serves as a motivation serves its purpose. The MacGuffin can sometimes be ambiguous, completely undefined, generic or left open to interpretation.
Alfred Hitchcock illustrated the term "MacGuffin" with this story:
"It might be a Scottish name, taken from a story about two men in a train. One man says, 'What's that package up there in the baggage rack?' And the other answers, 'Oh that's a McGuffin.' The first one asks, 'What's a McGuffin?' 'Well,' the other man says, 'It's an apparatus for trapping lions in the Scottish Highlands.' The first man says, 'But there are no lions in the Scottish Highlands,' and the other one answers 'Well, then that's no McGuffin!' So you see, a McGuffin is nothing at all."
...Hitchcock's verbal delivery made it clear that the second man has thought up the MacGuffin explanation as a roundabout method of telling the first man to mind his own business.