What the heck is RENNET?

Rennet is an enzyme derived from the third stomach of a ruminant animal (eg. cow, goat, or sheep). It acts as a coagulant, and helps the milk curdle faster (turn from liquid to solid). Not all cheese involves animal bits. There is vegetarian rennet available, and in the olden days most cheese was made with no rennet at all. The natural acidity of souring milk would cause curdling. However that takes more time than even the most patient cheesemakers have, and rennet is key to some recipes where you want a curd with a higher Ph.