Can someone explain nihilism more clearly?

Well, most nihilists are of the existential variety, meaning that they argue that there is no intrinsic value or meaning to life. The view is that there is no deity that created life, and therefore life is the result of happenstance and thus lacks any meaningful purpose.

But what you're talking about is moral nihilism. It's actually pretty logical - morality is a construct that we humans built to control our fellows, and it does not inherently exist. There is no objective morality because what someone views as moral is often not shared by another random person. Morality, thus, is entirely subjective, and therefore the idea that something is "good" or "bad" is also entirely subjective. So long as we agree that morality is subjective, nihilism makes a good deal of sense, since it's not so much a denial of good and bad, but rather a denial of an objective good or bad.