During the Colorado floods this summer, someone at NOAA started claiming that it was a thousand year flood. When that was shown to be utter nonsense, they changed the story to a thousand year rainfall, based on the odds of that much rain being received at a particular point location.
Suppose there 10,000 rain gauges in the country. The odds of any one of them receiving a thousand year rainfall in any given year are 1,000:1. So we would expect about ten “thousand year rainfall” events in the US every year (10,000/1,000.)
This is high school statistics, not climate change.