The Biological Value (BV) is a scale of measurement used to determine what percentage of a given nutrient source is utilized by the body. The scale is most frequently applied to protein sources, particularly whey protein. Biological Value is derived from providing a measure intake of protein, then determining the nitrogen uptake versus nitrogen excretion. The theoretical highest BV of any food source is 100%. In short - BV refers to how well and how quickly your body can actually use the protein you consume.
Whey protein isolate, which has the highest rating of protein created in a lab, is rated 159. The highest rated food are whole eggs, rated 100.