Buttermilk powder is dehydrated buttermilk. It is naturally low in fat and adds the creamy tang that is often found in ranch dressing. If you don't have buttermilk powder, you can still make the ranch seasoning but it won't have that classic buttermilk flavor.  If making dressing or dip, you can add liquid buttermilk if you have it or add some lemon juice to your dressing/dip to add some tang.