The tragedy of the commons is a situation in a shared resource system where individuals users, acting independently according to their own self interest, behave contrary to the common good of all users by depleting or spoilt the shared resource through their collective action.
Overfishing is an example of the tragedy of the commons because multiple people are fishing for their own gain disregarding the negative effect depleting fish from the ocean can have. In this case the ocean is the shared resource system being spoiled through collective action.
Hope this helps :)
