A chain is made up of a row of links fitted together. If all the links are strong, the chain will be strong also, but if even one link is faulty, no greater strain can be put on the whole chain than can be put on that single weak link. The proverb means that it does not matter how strong a team is, if they have a member that can be exploited in weakness, then they can all fail.
If a number of persons have joined together in some enterprise, the weakest link is he who lets down his companions by not playing his full part in the carrying out of their plans. For example, the conspiracy known as the Gunpowder Plot, the purpose of which was to blow up King James I and both Houses of Parliament on November 5th, 1605, the day fixed for the opening of parliament, failed because one of the plotters warned his brother-in-law not to attend the opening. Because of this weak link, the chain broke, and the Gunpowder Plot miscarried.
Another version of this proverb is: “A chain is only as strong as its weakest link.“