What does Kosa represent?

Kosa symbolizes sacrifice and gratitude for that sacrifice -- the preparation of kosa for the spirits and ancestors that Halmoni and Lily perform together is their way of showing gratitude for those who came before and for the sacrifice or struggles they experienced in order for Halmoni and Lily to be able to live as they do. The sacrificial symbolism is further emphasized by the fact that Halmoni and Lily cook actual food for the spirits and ancestors which they leave out for them before even preparing their own meal or beginning to eat. They sacrifice food and comfort to say thanks for the parallel sacrifices that came before.