Kratom is a leaf that is primarily farmed in Asia which contains the active ingredient Mitragynine, which has some very mild partial-agonist opioid effects. When kratom is consumed, your liver converts a very small amount to a MUCH more potent molecule called 7-hydroxymitragynine. As with all natural drugs, some asshats in a lab decided to figure out how to create the latter molecule semi-synthetically and in bulk. They pressed them into pills and sell them in head shops and some gas stations. It's pretty addictive, nasty stuff.