Some of your Steem rewards are in Steem Power (SP) which gives you voting influence. To get that into 'liquid steem' that you can transfer to others or use on markets you have to 'power down'. That transfers SP to Steem over 13 weeks. You can stop it at any time. Steem Dollars are supposed to be pegged to US$, but the price has been driven up lately. These would be useful for buying goods as you should know what they will be worth. If you search around you will find lots of tutorial posts. Also see the FAQ that's linked from the main menu (top right of the page).