Tangyuan is a type of boiled dumpling made from glutinous rice flour. It’s mixed with a small amount of water and shaped into round balls (yuan) and cooked and in sweet or savory soup (tang), then served immediately in a bowl. They come in a wide variety of colors and fillings from sugar to meat. They can also be fillingless, which are usually smaller in size. The most common, "default" one is white with sweet sesame paste.
Tangyuan are traditionally eaten during Dongzhi festival, Yuanxiao festival and served as a dessert on any occasions such as wedding day, family reunion ect. In modern days they can be served as casually as an ordinary breakfast meal.
There is a technical difference between "tangyuan" and "yuanxiao". "Tangyuan" refers to the southern style, whereas "yuanxiao" refers to the northern style. The two are primarily differentiated by their method of preparation, texture and size, though they don't look that different from the outside.