They are traditional, Cantonese, steamed dumplings. Usually, they're filled with prawns or pork, but there a wide variety out there. They're usually served with a soy-based or red chili sauce.
Dim sum is a variety of small bites, ranging from steamed dumplings to small dishes of traditional Chinese dishes. At Yen Ching, they have things ranging from steamed buns with bbq pork filling to my fave, the chicken feet.
As far as i know, it is a chinese dish of small streamed or fried savory dumplings containing various fillings. So dim sum in yenching menu refers to varieties of dumplings they offer. I hope this was helpful
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