Credits can be used, like mentioned above, to buy more player servers slots- which will get you more ram depending on the amount purchased, for this payments are daily. As well as this, you can purchase extra server slots for 400 credits each, which are a one time payment. Ranks can also be purchased for credits but you'll need to continue voting/buying them to have enough for them!
Check out this link to purchase ranks, and here to get more player slots!