Commodities are physical goods like grains, gold, oil, and so on.
Historically, commodities produced lower returns than stocks, but they offered diversification and some protection against inflation.
Unless you are a very sophisticated investor, you should avoid investing directly in commodities. It is also a good idea to leave derivatives for professionals. If you want to invest in commodities, invest using a commodities fund instead.
Commodities are not normally listed in an Investing for Beginners guide, and I agree... it is better used by more experienced investors. On the other hand, to present a better picture, I believe it is important that beginner investors understand all investing possibilities.