Dried, aged mandarin peel, soaked until soft before use. Cantonese cooks add it to steamed beef balls, steamed fish, congee and red bean sweet soup; it tastes sweet at first, then bitter.
陈 literally means 'aged' - the peel is valued more as it ages, and Xinhui 新会陈皮 from Guangdong can sell for premium prices after 10+ years of aging. Used in TCM to 理气 (regulate qi) and in Cantonese cooking for depth of flavor.