Double boiling is slow cooking ........ means to put a soup pot or any covered ceramic or steel pot inside a bigger pot, immerse in boiling water, and let the soup cook at a lower temperature. Hence the soup is cooked from heat generated from the boiling water and not from direct heat source.
Double boiling lets the soup ingredients slowly release their nutrients into the soup, thus making it tasty and wholesome. It's a long cooking process, roughly averaging 2 to 4 hours. The tip is not to open the cover to check on the soup as it'll bring down the temperature and affect the cooking process. The plus point is there's little evaporation and the soup will not boil over. The only thing is to monitor that there's enough water in the bigger pot and not let it run dry. Of course, nowadays, the slow cooker is another alternative to double boiling, saves the hassle of checking the water level.