Piperine is a naturally occurring compound that is found in black pepper. The piper nigrum plant provides black pepper. The peppercorns are picked just before ripening, fermented and left to dry until they shrivel and turn black. The piperine is extracted form these black peppercorns.
We know that the curcumin in turmeric is not easily absorbed by the body. In fact, research has shown that when curcumin is taken orally, very little actually makes it as far as the blood. We don't know where the curcumin is metabolised, we just know that not much gets through to be used by the body.
Studies have shown that when curcumin is taken alongside piperine the bioavailability (amount of curcumin that is available to be used by the body) increases dramatically.
These studies have been conducted on both laboratory animals and humans, with one study showing an increase of available curcumin in the blood of 2000%.