Look here :

https://community.windy.com/topic/3361/description-of-weather-overlays

It is also easy to find out for each layer. If you click on the map it will display the unit used for that particular layer. In your case, it's Cloud cover in percent. 0% is a brown area : sky is clear. In a grey area, the sky is overcast, entirely covered by clouds so it's 100%. Then if you drag the cursor on thoses crosshatched areas, it will display an other information with a new unit : rain/snow accumulation for 3 hours in millimeters (mm). It means that those areas are covered by clouds and moreover, it's likely to rain or snow there.