SteemPower gives you voting power, so the more SteemPower you have, the more weight it has when you upvote or downvote things; however, SteemPower is not transferrable so you can't send it to someone else or sell it like you could Steem. You can Power Up to turn Steem into SteemPower (this is instantaneous), and you can Power Down to turn SteemPower back into Steem (but this takes a long time).

If I remember the rules correctly, you can Power Down and not claim the Steem that is powering down until it's all done, in which case you'll get it all in one year, but every time you claim some of the powering down Steem before it's all powered down, it makes the rest of it take longer to finish powering down, so if you're constantly claiming every bit as it powers down, it'll take 2 years to get it all. So basically, you can power down quickly (1 year) by being patient, or slowly by being impatient (2 years) but you get some of it sooner.