Catching so many fish that there aren't enough adults left to breed and replace the ones you caught.
Because the population can't rebuild itself there are fewer and fewer fish in the sea. If people carry on over fishing it can result in a species becoming extinct.
Over fishing is also really bad news for groups of humans who rely on the fishing industry. If fish numbers get too low they lose their livelihood. That's why it's important to harvest the sea in a sustainable way, taking no more out than the environment can spare.