Hello,
The reward points are like on site currency. I will make a nice Information page soon.
Example
You buy a game for $24.99 you will get 25 reward points.
Also if you noticed a game that cost $24.99 USD cost 2499 Reward points. So in practice you can think of each reward point being worth slightly more than 1 US Cent.
You can apply reward points to any order at any amount, it doesn't have to cover the full amount. If you have enough to cover then you can just checkout for free and the order is processed like regular. Remember that if you used Reward points on an order you will not get any reward points on that order.