A 100% chance of rain means it will certainly rain in a particular area during a particular time, but it may or may not include your forecast zone or area. The nature of our atmosphere means it is not possible to predict the weather on a particular day months to years ahead.
100% chance of rain mesns only that for dure it will rain. In no way means that 100% of the time will be raining. You eill seldom see a forecast of 100% chance of rain, there is always uncertainty