When water or another liquid is roiling, it's stirred up and moving in a churning way. The roiling, soapy water in your washing machine tells you that your clothes are getting thoroughly cleaned.
If the ocean all around your little boat is roiling, it's full of waves, tossing you on its surface — and the roiling green liquid inside your blender might turn out to be a delicious smoothie, despite its strange color. The adjective roiling comes from the verb roil, "move in a turbulent way," which has an uncertain history. One origin might be the Middle French rouiller, "to rust or make muddy."