It's generally used to cook McDonalds food (mainly the fries, as they are submerged in it) and is what makes it so delicious. It's OK to eat some every once in awhile but can cause death (usually in the form of a heart attack) if enough is consumed. Eat at your own risk.
Joe: *starts eating McDonalds fries*
Dave: Stop! You're eating the McPoison!
Joe: These are the only fries I've eaten all week!
Dave: Alright then...