This is an acid base reaction. Baking soda is primarily composed of NaHCO3, or sodium bicarbonate. This is a basic substance and will have a pH above 7. Buttermilk has lots of fatty acids which are mild acids as their name implies. As such, the pH of buttermilk is below 7. When the two are mixed together, they will neutralize each other and the pH of the mixed contents will be close to 7 (neutral pH). The byproduct from this reaction is CO2 gas, so you will see bubbles emerge from the liquid as the CO2 escapes into the atmosphere.