How might the overuse of antibiotics lead to resistant strains of bacteria

Well your body could possibly become immune to the antibiotic after taking so much of it for a long period of time.

They represent deadly bacteria that have become resistant to antibiotics traditionally used to eradicate them

Using antibiotics when ineffective or unnecessary enables the mutation and spread of antibiotic-resistant bacterial strains, since the bacteria will get small and/or a short duration of doses of the medicine, too short to kill the bacteria, but able to make them get used to the "poison" and no longer killed by it. That is why you are told by medical professionals that you should always take all antibiotics prescribed even when you start feeling better, because it takes the full amount of medicine to be sure the bacteria have been killed. You are also told not to skip doses for the same reason.