@Mondeuse
Salut,
There is no real guide to explain how to use this App.
You can get information via this link
https://community.windy.com/topic/3361/description-of-weather-overlays

A member created a user guide some time ago
  http://www.maccinfo.com/Introduction_to_Windy.html
  But it has not been updated since that time.
  If you have special questions, you can ask them here.