What does Granger mean by his quote "You're not important. You're not anything."?

Granger is referring to the lack of importance most individuals have over time in the grand scheme of things. Each person's life is merely a few seconds in time and space. He adds that even great men are eventually forgotten,

"But even when we had the books on hand, a long time ago, we didn't use what we got out of them. We went right on insulting the dead. We went right on spitting in the graves of all the poor ones who died before us.
His point is that we don't do much with what we have while we have it. So before we get conceited and think we have life and everything around us all figured out, it is important to recognize how small we are in the large scheme of things. On his own, Montag isn't much. But the power of the group could be tremendous if each one did their part."