Bioavailability refers to quantifying the ability of the human body to extract from a substance the nutrients or other chemicals it needs for a particular function.
Deeper dive
Simply ingesting a drug or supplement does not necessarily mean it will interact with the body. If the drug or supplement is intended as treatment, it needs to be bioavailable: that is, some part of it must actually enter the bloodstream and be used by the body for some kind of function. Bioavailability, often expressed as a percent, refers to the proportion of the substance that the body can absorb and use.
For example, taking vitamin D supplements may not actually increase the body’s vitamin D levels if the supplements are not taken in a bioavailable form. Bioavailability can also be applied to contaminants, such as the bioavailability of lead in contaminated soil.
Although bioavailability requires absorption, absorption does not necessarily mean something becomes bioavailable—it has to survive the digestive process. For the nerdy specifics, this link discusses the equation in which bioavailability and absorption are different variables.