It means that the refrigerant has leaked out of the seals in the system. This happens if the a/c is not used regularly. For example, it’s best to use it every week through the winter because the refrigerant carries lubricants suspended in it. After having been regassed the system may or may not remain gas tight, so it would be sensible to ask for a dye to be added so that the point where it is leaking can be identified. A/c FAQ here: www.honestjohn.co.uk/faq/air-conditioning. A domestic refrigerator does not live in the same hostile environment as a car, which has to cope with temperatures from minus 30 to plus 50: ice, snow, rain and sun.