Traditionally it’s what left over after turning milk into butter and letting it sit to ferment and thicken. Another of milks delicious byproducts.
Without milk we wouldn’t have cheese, whipped cream, buttermilk pancakes or buttermilk fried chicken.
Milk is such a great ingredient as is buttermilk. Nowadays buttermilk is just milk that gets fermented with the same cultures used to make yogurt.