Origin
The proverb has been traced back to C. Kingley’s letter dated December 1, 1856. It first cited in the United States in the early twentieth-century. The proverb is found in varying forms: “A chain is as strong as its weakest link”; “A chain is only as strong as its weakest link”; “The strength of a chain is its weakest link,” etc.
Explanation
“A chain is only as strong as its weakest link” applies to any process that will fail if some step in it goes wrong. For instance, a building isn’t well protected if its security system has one flaw—say, an easily pickable lock—that allows unauthorized entry, no matter how elaborate and expensive the rest of the system is. An organization (especially a process or a business) is only as strong or powerful as its weakest person. A group of associates is only as strong as its laziest member. A chain is no stronger than its weakest link. No matter how strong someone or something is strong, it is always limited by its weakest attribute. A successful group or team relies on each member doing well.This means that processes, organisations, etc, are vulnerable because the weakest person or part can always damage or break them.
Example
George is completely out of shape. I don’t want him on our ball team; a chain is only as strong as its weakest link.