Natural describes something that comes from nature, rather than being man-made. Your healthy friend who only eats natural food will probably choose carrots instead of potato chips for a snack.
The adjective natural is a common word with a lot of meanings. It describes anything that comes from nature, but it also means "inborn" when you describe your basketball-star friend as a natural athlete. You can also use it to talk about someone being at ease ("he tried to act natural when the police passed by") or to mean "inevitable," as when you say, "It's only natural that those two ended up getting married."