A silver bullet is a term that tends to express a straightforward and simple solution to a problem. It has origins in the lore that werewolves could be killed with an actual bullet made from silver. Earlier, it was thought witches and vampires could also be killed this way, though the more traditional destruction of witches was burning and, traditionally, both beheading and staking a vampire through the heart is thought to be the means by which one is dispatched.
Some technological developments have been called a silver bullet. For example, the phrase is often used to describe the discovery of antibiotics and penicillin, in particular, because of the many diseases it could treat. Often, the term is used to describe the cure all for a problem threatening humanity.
Because a "silver bullet" is a wide-reaching cure, something that's extremely rare, the term is frequently used in the negative to dismiss something as not easily accomplished. For example, someone might say that “increasing funding to schools is not a silver bullet for improving education.” Often the negative phrase is more common than something actually being described as a silver bullet.
This is likely because there are very few simple solutions in a complex world with complicated issues. Direct, straightforward solutions cannot always solve difficult problems. Thus, for example, some look to cloning human organs as a potential silver bullet for the lack of available organs needed for transplant. This is by no means an easy answer, however, because such cloning is still far from perfect and both medical and ethical questions remain.
The silver bullet solution to address an issue is often a matter of wishful thinking. In hindsight, even a medication like penicillin has created its own issues. Medical researchers know they now run a race between developing stronger antibiotics and between creating more strongly antibiotic-resistant strains of bacteria by applying stronger antibiotics. So while the silver bullet seems like a splendid concept, it oversimplifies the complexity of any problem.