Hello and Welcome!

• I don't even remember this is Beta because it's too good to be true.
• Steem Power is steem but not liquid. You cannot transfer it or trade it immediately. In a way, it's like the stocks of the Steemit Inc.
• Yes, you can convert it. Steem to Steem Power = Power up.
  Steem Power to Steem = Power down.
  If you decide to power down, you'll get it out in 13 weekly payments.

Why Steem Power? And What for?
The more Steem Power the more influence you have = the more $ your upvotes will originate. Right now your are upvoting everything but when you have a reasonable amount of SP you will notice your upvotes will give more money to other users. That's when you might want to become more selective.
SP always influences your curation reward, so your upvotes not only are better for others but also for you. In short, this is why it's a good idea to have a lot of SP.

In Steemit's point of view, it represents the users commitment to the platform and avoids people selling large amounts of Steem for whatever reason (that would drive the price down).

Cheers!