Xiaolongbao, also known as soup dumpling, is a type of baozi (filled bun or bread-like item) from Eastern China, including Shanghai and Wuxi. These buns are traditionally steamed in bamboo baskets, hence the name.
Xiaolongbao are traditionally filled with soup and meat, but variations include seafood and vegetarian fillings, as well as other possibilities. The soup inside is created by placing some meat gelatin inside the dumpling before steaming. The steam heat melts the gelatin into soup. In modern times, refrigeration makes it easy to wrap up using chilled gelatin which otherwise might be liquid at room temperature during hot weather.
Traditionally, the Xiaolongbao is a dianxin or snack item. The bun is served hot. It is dipped in Chinkiang vinegar with ginger slivers, and is traditionally served with a light, clear soup.
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参考资料：Wikipedia