That is Steem Power that's been "delegated" to you. Think of it as a loan of Steem Power from somebody else. Likely in your case, it was 'loaned' to you temporarily by the main Steemit account...for free.

This delegated, or loaned Steem Power acts as if it's yours when you vote...so your vote gives more $ when you cast it. However, you can't actually withdraw that loaned Steem Power since you don't actually own it.

I hope I didn't make it more confusing...but I'll put a guide together on this topic. :)