Ayahuasca is a Quechua word for a vine, Banisteriopsis caapi, and it is also the name given to the many different brews made from that vine. When somebody talks about drinking Ayahuasca they mean the brew, which is the vine cooked for hours with at least one other plant. Recipes vary enormously across the Amazon, so there is no single thing called Ayahuasca. What every version has in common is the vine.
The vine contains compounds that briefly switch off an enzyme in the digestive system. That enzyme would otherwise break down dimethyltryptamine, DMT, which is what the companion plant supplies. So the combination is what makes the brew orally active.
Why the Tradition Puts the Power in the Vine
It is common to read that Ayahuasca is a way of getting DMT past the stomach, and the people whose medicine it is do not describe it that way at all. To them the power is in the vine. The vine on its own carries no DMT and is worked with on its own in some traditions, which is difficult to square with the tidier chemical account.
I am not going to try to settle that here. I am telling you what the tradition holds, because it is the tradition I was taught in, and because a reader deserves to know that the pharmacological summary is not the whole of what is being claimed.

