Red moong is a variety of mung beans. Called Adzuki beans in Asia and Chori Dal in some parts of India. They're small whole red beans - just like a green mung bean, only orange to red in color. When cooked they have a deliciously creamy/soft texture, while still holding their shape.
That makes them great for the sauce they stay in shape and unlike other, puree style dal dishes like my delicious Parippu (made with yellow moong) have a chunky texture.