Your STEEM are locked up as Steem Power. Powering Down converts them back to STEEM in weekly batches, over a period of 2 years.

Steem Power gives you voting power, so the more Steem Power you have, the more weight it has when you upvote or downvote things; however, Steem Power is not transferable so you can't send it to someone else or sell it like you could Steem.

You can Power Up to turn Steem into Steem Power (this is instantaneous), and you can Power Down to turn Steem Power 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.

https://steemitimages.com/DQmf4uhg8NTZj5hbzFUKRg1Gw592jKi8sU3jkYmcKoEZyEX/image.png

You can power up and your committed for 2 years but your not subject to the Steem dilution. You can Power Down and you'll get a withdraw every 7 days based on when you hit the button (or you can cancel at any point). The withdraws are equal but will you will see a bit more Steem each time to compensate for the extra Steem in print. The Web UI does a full power up and power down (all Steem or all Vests respectively) although the blockchain supports the power up or down of any amount.