It is a noun.
Meaning: A small weakness or fault in a person or a system that can lead to its failure.
Origin: Achilles was dipped in a special water for making him eternal . But the heel was not covered by water. This led to his death in his later life. This signifies that even strong heroes like Achilles, if have weaknesses, can fail. That weakness is thus called as 'Achilles heel'.
The corrupt minister was considered as a government's Achilles heel and was asked to resign.