When we say nothing, we mean "aught," "zero," "a big goose egg," "nada," "nil," "zilch," or "zippo," in other words, "of little or no quantity."
Nothing can mean "not having anything." You might be hungry because you had nothing for lunch. It can also mean "not a big deal." That's why if you thank someone for doing something for you, he or she might reply, "Oh, it's nothing," to mean the effort was not at all significant. Nothing can also mean "emptiness" as in, "There's nothing in the trunk of my car."
