When you Power Up, you are essentially converting your Steem or Steem Dollars into Steem Power. The best and easiest way to look at it is this: You are paid by Steemit in the form of Steem for curation rewards and Steem Dollars for authoring rewards. Steem Power is how you re-invest that money into Steemit to increase the weight of your upvotes and comments and your value in the site. Keep in mind however, that converting Steem Power back into Steem takes 104 weekly payments, so a very long time (called Powering Down)! Be sure investing your money into Steem Power is a good choice before you make it!