"Bioavailability" is the ease with which any nutrient can make its way from the food you eat into the body. The "journey" that every nutrient takes is the same. In the first part of the journey, the food must be digested (broken apart) so that the nutrient can be freed from the food that contained it. Once the nutrient has been freed from the food that contained it, the second part of the journey involves absorption of the nutrient from the digestive tract into rest of the body. When a nutrient is highly "bioavailable", such such as those found in raw and real food, it can be digested and absorbed a high percentage of the time and in a dependable way. When a nutrient is poorly bioavailable, its digestion, its absorption, or both can be much more difficult and much less predictable (see: Wikipedia).