A chain is only as strong as its weakest link
The proverb has a literal meaning, although the 'weakest link' referred to is figurative and usually applies to a person or technical feature rather than the link of an actual chain.
We are most likely these days to come across the phrase 'the weakest link' in reference to the popular TV quiz show of that name, which originated in the UK, hosted by Anne Robinson, and was later syndicated for use in many other countries. The show, which relies on the demonstration of the abysmal lack of general knowledge by many of the participating contestants, is an example of the many 'humiliation television' shows of the early 21st century and is a sad spectacle.
It is clearly a literal fact that a chain is only as strong as its weakest link. The conversion of that notion into a figurative phrase was established in the language by the 18th century. Thomas Reid's Essays on the Intellectual Powers of Man, 1786, included this line:
"In every chain of reasoning, the evidence of the last conclusion can be no greater than that of the weakest link of the chain, whatever may be the strength of the rest."
See also: the List of Proverbs.