Buttermilk is a dairy product, the liquid part left behind after churning butter out of cream. Sometimes, two species of bacteria can be added to it, so producing a fermented milk drink, giving it a sour taste. This drink is quite common in warm climates, and also in northern European countries.
Traditional Buttermilk is considered beneficial to health (above all for gut and immunity system, but also stomach and skin) as it contains probiotic microbes. Furthermore, it has a quite low fat content, and also is a source of vitamins, potassium, calcium, and even phosphorus.
In cooking, it is an important ingredient to make many American desserts, and fried chicken too. You can make it at home, just leaving milk turn sour, or by combining milk and yogurt, or even by whipping cream till the fat droplets will stick together destroying the colloid and forming butter and buttermilk.