Comment by sweettea

My understanding is that it's an expected value based on coverage in each of the ensemble scenarios, not quite as simplified as "how many scenarios was there rain in this forecast cell".

At least for the US NWS: if 30 of 100 scenarios result in 50% shower coverage, and 70 out of 100 result in 0%, this is reported as 15% chance of rain. Which is exactly the same as 15 with 100% coverage and 85 with 0% coverage, or 100 with 15% coverage.

Understanding this, and digging further into the forecast, gives a better sense of whether you're likely to encounter widespread rainfall or spotty rainfall in your local area.