奶 is milk, and 黃 is yellow, so 奶黃 is milk yellow or custard, and 奶黃包 is custard bun.
About This Dish
Steamed buns with a custard filling. Strictly speaking, the custard in this bun is thick, almost crumbly, and the molten custard variety goes by a different name, 流沙包, but 奶黃包 is often used to refer to both versions.