A silver bullet is a specific solution card. It is incredibly powerful against a certain type of card or archetype, but useless or inefficient otherwise. An example of a silver bullet is Moat, because it stops almost all creatures, but is useless against flying creatures and decks that do not use creatures to win (such as Burn, Combo and their variants). A hoser is a particular subtype of silver bullet.
The term is used fairly widely depending on situation. If your deck only contains one answer to a certain situation, then that card is your silver bullet. If there are many cards in a format that significantly damage specific decks but one is significantly better, then that is a silver bullet. Often these cards find a place in side boards as a meta-game choice or as a wishboard.