Can someone help me understand what "Power Up" / "Power Down" means in the Wallet section?

Apologies for the noob question, but I have three:

• I just purchased some Steem and I see an option that says "Power Up" like the image below. I also see "Power Down" next to the Steem Power dropdown. What do these mean exactly?

• What is the "Estimated Value" in the bottom of the pic being derived from? It says Estimated Value of $33.44 but my Steem alone should be worth more than $60

• What's the different between Steem and Steem Dollars, if I want to trade/speculate on the price should I just deal with Steem?

Thanks