This is the ingredient in a medicine or treatment that is biologically active, meaning it has some interaction or effect on cells in the human body.
Colloquially, it seems to mean “the thing in this medicine that actually does something,” as opposed to other aspects of a medicine that are related to presentation, packaging, or delivery.
(Related to this, my wife gave me a “natural painkiller” she bought at the health food story. I looked on the ingredients, and it had nothing listed as an “active ingredient.” It was just a bunch of inert vitamins.)
Why I Looked It Up
I’ve seen the phrase for years in drug advertisements. I finally decided to look it up.