Nothing is pretty straightforward as far as titles go: it's the first word of the book, and we learn what it means on the first page. It comes from Pierre Anthon's declaration that nothing matters, which the rest of the class will spend the rest of the book attempting to negate. It's also part of the central tenet of existentialist philosophy: nothing can be understood except through the lens of individual experience, therefore all meaning is subjective. In other words, what means a lot to you might mean nothing to someone else.
The great existentialist philosopher Jean-Paul Sartre wrote a famous book called Being and Nothingness, in which he said that all human beings start as nothing and become only what they make of themselves. Pierre Anthon, of course, has decided exactly what he will make of himself: nothing, because there's no point.
Because, you see, Pierre Anthon has gone beyond existentialism (things do have meaning, but the meaning is based solely on individual perception) to existential nihilism (the individual is insignificant, therefore his perception is meaningless, therefore nothing matters.)