You can earn MRP's in a variety of ways. You receive them when making purchases, being a member of Wave3 and can also win them in the daily grand.
They can be used to make purchases at TripleClicks. You can buy anything from TCredits to eBooks to electronics.
I personally use them to buy TCredits. When bidding, you will receive 5 to 10 more MRP per bid. Look for the Double MRP auctions to maximize your MPR earnings!