If something fills or changes shape a lot with wind or air, it billows. If you hang out sheets to dry and a wind comes along, the sheet would billow in the wind. Sails on a boat obviously billow, because that is their purpose. If something gives the appearance of billowing - rolling or curling - we say that it billows. Clouds, white and fluffy, can be said to billow - especially, of course, in a wind.