This is officially one of the fluffiest food labels out there. There's so much confusion over what it means that the FDA is looking at whether or not it should define it more formally. The agency acknowledges the problem posed by processed foods, though: "From a food science perspective, it is difficult to define a food product that is 'natural' because the food has probably been processed and is no longer the product of the earth. That said, FDA has not developed a definition for use of the term natural or its derivatives." When you're in the grocery store, you can basically assume this word is meaningless.
"Good source of" something you should have in your diet
A cereal bar that’s a good source of fiber isn’t going to make you really healthy, or even make you poop well. "Good source" can actually mean that a food has as little as 10% of the daily value of the nutrient, in which case you’d need to scarf down ten of those babies to meet the recommendation. This would get you close to your calories for the day and constitute the opposite of a balanced diet, plus some bathroom issues, probably. When something is "high in," "rich in," or an "excellent source" of something, on the other hand, we’re talking at least 20% of the recommendation -- a little better, but still pretty bogus.