A braising mix is any miscellany of dark leafy greens, including kale, chard, spinach, mustard greens, turnip greens, beet greens, etc. Native Son's particular mix often includes kale, Mizuna, Tatsoi, and Dragon Tongue. Most of these greens are members of the brassica family, and so are rich in vitamin K, vitamin A, and vitamin C.
Despite it's name, a braising mix is not only for braising, but it can be stir-fried, sautéed, steamed, or added to soups or quiches. One of my favorite uses is to de-stem the greens, then tear them into smaller pieces before sautéing in olive oil and garlic and tossing them over pasta.
Check out this quick how-to for braising your greens and the recipes below, which were written specifically for braising mixes. But remember you can substitute braising mix in just about any recipe that calls for cooked spinach, kale, chard, collards, turnip greens or mustard greens.