Buttermilk was, at one time, the liquid left after butter had been made from milk. Unless you know someone at a dairy, this is no longer available.
Today's buttermilk is a cultured milk product, where a bacteria similar to the bacteria used to make sourdough processes the milk, making it richer, giving it a deeper taste, and making it more acidic.
Buttermilk is used in most quick breads because the acid in the buttermilk reacts with baking soda to produce gas and a rise.
You can use powdered buttermilk or soured milk in place of buttermilk to get acceptable, but inferior, results.