"soup song" means a heartbreak song
So why (is) this kolaveri a sensation? Kolaveri means "killer rage" and in the context of this song it just means, "Why are you so angry with me, girl?".
For a lay man, the beginning "Yo boys am sing song....soup song...flop song", Tamil-English mix and the lazy Koothu beat make it an instant catch.
According to Dhanush, "soup song" means a heartbreak song -- instant connect, isn't it? Soup to sabka banta hai boss!