In days yonder, buttermilk was the liquid left over when butter was churned. (You can get the same thing by running cream in a food processor until the butter solids separates from liquid a.k.a. buttermilk.) These days, the buttermilk you buy in the grocery store is made by treating milk with lactic acid to ferment it. Essentially, buttermilk is spoilt milk.
Buttermilk is actually a pretty common ingredient in baking. It has a low PH (read: acidic), so it tenderises confections like muffins and cakes.
I don’t know about you, but I have no other use for buttermilk except for baking, so it’s not always readily available in my kitchen.
So, my quick fix is to add 1 tablespoon of an acid like lemon juice or vinegar to a cup of milk and let it stand and thicken up for about 10-15 minutes. And voilà, a very serviceable buttermilk.
(Or, just use yoghurt in place of buttermilk like I so often do.)