Sour milk is milk that has naturally soured or has had an acid like vinegar or lemon juice added to it. It’s used for some baking recipes and is often used as a substitute for Buttermilk. See this recipe to make some: How to make sour milk.
Buttermilk used to be the liquid left over after making butter but today the Buttermilk you buy in the store is milk that has a bacteria culture added to it and allowed to ferment. This is what gives if the tangy flavor.