An antibiotic will kill bacteria. Most bacteria. Unfortunately, the ones not killed will have little baby bacteria that also know how to survive this antibiotic. Over time, each antibiotic goes from being extremely effective to nearly useless because bacteria have evolved resistance.

No matter what we do, an antibiotic doesn't last forever. There are some really foolish things we're doing, however, that are letting bacteria evolve resistance much faster than they have to. The biggest thing is prescribing a particular antibiotic when it is not called for. Antibiotics are on a continuum from the ones for which there is widespread resistance to the super powerful new ones we save for the most dire cases of multi-antibiotic-resistant disease strains. When treating a patient with antibiotics, the right thing to do is to use a low tier antibiotic unless there is evidence that this will be insufficient. Doctors, however, are responsible for individual patients, not global health, and suffer no penalty for prescribing an antibiotic more powerful than required. This dynamic leads to prescribing antibiotics that we should instead by holding in reserve, visibly increasing outcomes for patients but invisibly increasing resistance. The long term result is more people dying.

Patients are also at fault here in at least two ways. Because antibiotics are so effective when used properly to treat bacterial infections, patients demand them when they are suffering from viral infections upon which they will have no effect. Doctors can sometimes convince their patients that this will not help, but others capitulate and the patient gives whatever bacteria are around a chance to evolve better resistance to whatever antibiotic was prescribed. The other big problem with patients is not finishing the prescribed dose. Patients take the antibiotics as directed, but stop taking them once they feel better. This doesn't make them get sick again, but because not enough bacteria have been killed they can go on to infect other people. And it's not just any bacteria going on to do that: it's the ones most resistant to the antibiotics the patient was taking.

We also use antibiotics in factory farming, often relying on them to keep the animals healthy instead of giving them enough room or proper sanitation. I've heard people claim this is increasing antibiotic resistance, but I'm not sure how much of a problem this is because we're only feeding the animals low-tier antibiotics, not daptomycin.

While we do continue to develop new antibiotics, we're slowing down. Looking at wikipedia's list of when antibiotics were released, we can see that new releases are falling:

Future medical breakthroughs may save us, but there's a good chance they won't. We need to stop wasting valuable antibiotics on conditions that do not need them, and make sure future generations get to live with antibiotics too.