What does the weather forecast mean by 90% rain?

1, it will rain 90% of the time, and 10% of the time will not stop.

2. It will rain in 90% of the forecast area, and it will not rain in 10% area.

This number is used to measure the probability of rainfall, which usually refers to the probability of precipitation in a specific area at a specific time. But the trouble is that the exact meaning of probability is not fixed, so it has always been the focus of debate. One view is that probability should be a representation of the possible frequency of an event.