A macguffin is a term from the movie industry. Aa script writer came up with it to describe a standard plot device used in many stories. It is the super awesome magical item that will unravel the whole plot, save the princess, the universe, and fix everything. The protaganist will generally find it right before the climax, wrestling it from the antagonist, and saving the day.

I like to use the term rather more generically to describe any quest item that the player is sent to go (rather arbitrarily) collect several hundred of. The item itself has no inherent value, and will serve no purpose *other than* to fulfill this singular quest requirement. It is this particular point that links it to the script writer's macguffin. The protaganist is not actually going to use the ancient idol for anything other than to unlock the secret cave. It's useless otherwise (ignoring the fact that it's probably made of gold or something).