As has been stated, it is a favorite argument used amongst creationists to claim that some organic systems are too complex to have evolved as such.
In other words, it is the argument of "I don't understand how it works, therefore it's wrong."
Ironically, it can be used as an argument against God, too.
Creationist: "You can't explain how an eye evolved, it's too complex to have sprung up on its own. It's irreducible complexity."
Scientist: "Of course it can, you're just incapable of understanding the concept that it takes millions of years for features to evolve in to the things we see today."
Scientist: "Fine, please tell me exactly how your 'God' made the eye."
Creationist: "He made it in his own image."
Scientist: "How exactly did he do it, lay out the steps for me on the precise methods used."
Creationist: "I don't know...."
Scientist: "Thus, by your own logic, you've just disproved God. Fantastic job, let me buy you a beer."