It depends on where you live. According to The Straight Dope it is typically used to mean probability it will rain compared to historical data. But in some places it is used differently, especially where the weather conditions permit more definite rain forecasts. I live in Florida and there is rarely a question of whether or not it will rain but rather a question of how much geographical area will receive rain. During the wet months the probability of any rain is very high, so it's only a matter of how much area is going to get hit. During the dry months, the only rain we receive typically comes in the form of fronts moving down from the north, so again chances of any rain tend towards 1 or 0 while the coverage can vary.