Michael Pollan has said this in a multitude of interviews: the sum of his years of research has revealed that the secret to a healthy diet can be summed up in 7 words: Eat Food. Mostly plants. Not too much.
'Food Rules' is written as a kind of primer that discusses these 3 basic concepts in practical ways for any person.

To eat food - avoid 'edible foodlike substances' - highly processed concoctions designed by food scientists, consisting mostly of ingredients derived from corn and soy that no normal person keeps in the pantry and contain chemical additives with which the human body has not been long acquainted.

As for what to eat - mostly plants - scientists may disagree on what's so good about plants, but they do agree that a plant-based diet reduces the risk of dying from all the Western diseases. In a plant based diet, you consume far fewer calories while getting more of the good stuff that scientists continually cite - like antioxidants and fiber.

Finally, eat less. Spend more for quality food that tastes - and IS better, and you will need less of it to feel satisfied. Choose food experience over mere calories.

For more information, Pollan's other book is 'In Defense of Food' and discusses the science and experiences that led him to these three basic principles.