"Vol tarief" is the normal amount you have to pay.
"Reductie" or more commonly "Met korting" is a 40% discount on the price of the ticket. It's unlikely however that you can use this one.
Domasla is sort of right but on the other hand he isn't. Students with OV-card (OV = Openbaar Vervoer = Public Transport) can travel for free so we don't have to pay anything. If you take someone with you, the other person gets a 40% discount. However, during the weekend students can't travel for free. Instead they can only get the 40% discount.
There are some other cards that you can use, such as a "Voordeelurenkaart" which gives you a 40% discount. (Anyone can buy one of these for €55 a year, so it's not just for students or elderly.)
But I suppose you're not actually living in the Netherlands so you just have to pay "vol tarief".