A chain is composed of a series of small objects connected together in series. Each of those objects is called a "link". "One of the links" means "one link". Next time you get chance, take a look at a chain, any chain at all, and see if you can identify each of the links that together form the chain. A chain isn't a tangible thing. All that there is to a chain is a bunch of links. A chain is just what we call "a bunch of links connected together" for short. :)