Hi @yuanbao ;) welcome to Steemit I'm from Belgium maybe I can explain you a little about SP, power down etc...

SteemPower: More bigger it is, more power voting (and give/bring rewards) you have on post (i.e. "whales" have big SP on wallet sometimes 1.000.000 SP when they vote on a post they give big reward ^^ my SP is 3200 when I vote on a post I give 0.001$ reward)

Power Up: convert your Steem (tradeable token on exchange, Bittrex, poloniex...) into SP (not tradeable)

Power down: Convert SP into Steem to trade them ;)

I don't understand the savings in the wallet hope someone can explain

If I can suggest something to you for the beginning use the 100% power up in your post your account will become bigger faster.

If you have any questions ask me ;)
Steem on!