Endogenous cannabinoid means that it is a chemical produced by our bodies that acts on the CB1 and CB2 receptors. Tetrahydrocannabinol exerts its effects by mimicking anandamide, which is also found in chocolate with some other chemicals called the N-acylethanolamines that prevent anandamide from being metabolized.

Similarly endorphins are the endogenous opiates if that helps any.

http://en.wikipedia.org/wiki/Anandamide