That's the Weekend Vrij-subscription. It means unlimited travel by train in the weekends (Friday 6.30pm - Monday 4am) for € 31,60 per month.

For € 3,10 extra you also get 40% discount during off-peak hours on weekdays.

Just read 2 lines on-and all info is there, also in /en/ in the general site.

There is also DalVRij= Off peak unlimited, cost about 107€/month, that means any day, but on normal working days mo-fr ONLY for check in in off-peak times. Valid on both NS and the various other regional trainlines-NOT bustrammetro!