You may have heard this term thrown around lately. Nutrient dense foods contain the micronutrients; vitamins, minerals and trace elements that we need for optimal health. Consume these foods and they will help your body to acquire more: Vitamins A, E, K, C, B, Folate, Zinc, Magnesium, Omega 3 Fatty Acids, a wide range of antioxidants, and micronutrients to help support your liver and detoxification systems, as well as your immune system.
That's a lot to keep track of and most of us don't have the time. Basically you want to focus on food that is minimally processed and close to its natural state. Here's a couple of links to nutrient dense eating from around the web:
Surf Nutrition: The Most Nutrient-Dense Foods