Steem Power is a digital representation of your commitment to the community. You earn it with popular posts that get upvoted. If you cash it out--powering down--then you take profits for yourself, and your SP drops. If you don't, your SP builds. The better author you are, the more SP you collect. The more of that SP you leave in your wallet, the higher your voting power, because leaving the Steem Power there signals your commitment to the health and stability of the community.

Hope that helps.