It means Steem Power (SP) and you see it when a person is Powering Up. So, you can go into your settings on http://Steemit.com and choose how much money, AKA the rewards, goes to Steem Backed Dollars (SBD), Steem, and SP. Normally, when you make money on Steemit, 50% goes to SBD and Steem and the other 50% goes into your SP. You can see it all in your wallet on Steemit. You can edit your payout plan in Settings. You can choose 50% or 100% to be sent to SP when you make money on Steemit. When you choose 100%, then you may see the Steemit sign, and it is called Powering Up. And more SP means more voting power and more Resource Credits (RC) transaction manna power which you can see at http://steemd.com