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. :)

你好 David！ "One of" 的意思是 “（某某）之一”。"Chain" 就是链子。链子里的每一届（每一个椭圆的铁圈子）就是 "link"。这句话的意思是：让伦子行动的链子里一届断了。

If you picture a chain, it is a string of overlapping circles connected to each other. The bits with the circles loop through one another are the links. So this sentence describes the situation when a lot of pressure is put on the chain or on one connection in the chain and that point breaks, leaving you with two shorter chains. It's often used metaphorically to describe a process involving many stages going wrong due to a problem at one stage.