"Bioavailability" is the ease with which any nutrient can make its way from the food you eat into your body. The bioavailability "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," 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.
Typically, we don't have to worry about the bioavailability of nutrients from our food if we eat in a healthy way and have healthy body function. The Healthiest Way of Eating that is described in my book, The Healthiest Way of Eating and on the WHFoods website (www.whfoods.org) is a way of eating that should free you from concern about nutrients and their bioavailability, unless you have special health issues that require nutrients in amounts that accommodate your own biochemical individuality.