2..what does it mean for the "Dvelopers of paid version can decrease opacity of Windy logo to 0.8 (however logo must remain clickable)
When using the free version, the Windy Logo has to stay at 100% opacity, but if you are using the paid version you are allowed to make it more subtle by dropping the opacity by as much as 20% (down to 0.8). By default, clicking the logo will take you to Windy's site... this has to stay the same regardless of paid / free version.
and usage limit is average of 5.000 Unique Visitors a day (measured by Google Analytics)" from the paid version description ？ does it mean can only be used by five people per day ?
That means you can have an average of 5,000 (five thousand) unique visitors per day.