It is for all four of five entire weekends within the month. Note that it's only for trains and not for bus, tram and metro.

thanks, i get it, but qua trains, all train networks in NL, so also Arriva, etc

Yes all train networks including. Not valid on Eurostar and Thalys. Valid form friday 18.30 hours untill monday 04.00 hours

https://www.ns.nl/en/nsflex/webshop#/bestelling/producten/4 

thank you for all the help so far, i see this note under this abonnement, what does it mean? can i travel 1st class - is it included in the price, or is it to be paid separately? 

By default, your Weekend Vrij subscription will only provide free travel during the weekend in 2nd class. You can add a supplement to als include 1st class travel.

The screenshot you sent is an option to switch to change your travel class to 1st class, which can be done until 15 minutes after checking in. If you don't have the supplement, this will also change the travel class for your trips during the weekend, but you will pay extra for those.