On Naming and Necessity (1980).
What's the relationship between language and the world? Specifically, what makes a name or a class term (like "tiger") pick out the person or things that it does? Saul Kripke wanted to correct the dominant view of his time (which involved speakers having some description in mind, and it's that description that hooks the word to the thing), and used modal language to do it: He talked about other possible worlds (other ways our world could have turned out, not literal other dimensions or something).
His account had implications for metaphysics and science, in that he claimed that if we find a scientific truth like "heat is the motion of molecules," then this would be true in all possible worlds. We might think that we could have discovered that heat was something else, but really, if we imagine a world in which that happened, what those scientists would have been looking at was actually not heat at all.