In other words, the system or machine has to have all of its parts present and operating in order for it to work, but, if this is so, it's a mystery how such systems could have evolved gradually by chance over long periods of time. Early versions of the system would lack parts that hadn't evolved yet and thus the prototype would be culled out by natural selection and thrown into the evolutionary wastebasket.
Various attempts have been made by Darwinian scientists to answer Behe's challenge to describe a plausible pathway by which these systems could have evolved, but few of them are persuasive and all of them are purely hypothetical.
In the process of promoting his idea of irreducible complexity Behe made famous the exquisitely tiny nano-machines that exhibit IR in the cells of all living things. Now a documentary has been made about Behe featuring his life and work. It's titled Revolutionary: Michael Behe and the Mystery of Molecular Machines, and the trailer of the film features a description of the most famous of the bio-machines, the bacterial flagellum, and also illustrates several others. Take a look:
The more one reads about the structure of life the more one is awed by the marvels one reads about and the harder it is to suppress the intuition that these things are designed. Indeed, Nobel Prize winner Francis Crick acknowledged the problem when he wrote that "Biologists must constantly keep in mind that what they see was not designed, but rather evolved."
Of course, if one has a prior commitment to naturalism then design by an agent simply doesn't fit into one's grand narrative and will be rejected out of hand. If one is more open-minded, however, the intuition that these things are designed is virtually irresistible.