These are mustard leaves that have been pickled in brine, sometimes with chile. The leaves have a slightly sour, salty taste and are used as a condiment or flavoring agent. Make sure to rinse them well before using to remove extra salt. Don’t worry if you can’t find these exact pickled greens at an Asian grocery store. There is a seemingly endless variety of Chinese pickled greens sold in packages, and most vary only slightly in taste and will work just fine in most dishes. These are also called preserved mustard cabbage, and preserved vegetables.