The difference between a substance that's bioactive and one that is bioavailable is best illustrated with an apple.
If you ate the whole of an apple your body would absorb some of it but not all (e.g., the pips, stalk and fibre). That's bioavailability, whether you can absorb what you've eaten.
Also there are bits in the apple (e.g., sugars and vitamins) which make things happen in your body . These are bioactive substances - active because they do things like give you energy, and keep you healthy.
Good supplements are both bioactive (do something) and bioavailable (absorbable).
You can read more about bioavailability here: