It's what we call "Voting Power" here on Steem. It's simply like an "energy bar" in which it goes down a bit everytime you made a vote(upvote/downvote).
Everyone starts with a full 100% Voting Power. Every time you made a vote (upvote or downvote) your VP goes down with respect to the upvote weight you give. If you upvote at 100% vote weight then 2% is taken from your VP.
Voting Power regenerates at a 0.8333% per hour or 20% per day. That is why it is recommended to only give a full upvote 10 times a day so as to not to deplete much your VP.
It is there to show the percentage of voting power you have left. every vote you place the line shrinks. You start at 100 percent and are able to do 10 votes a day which will recharge every 24 hours. You can do more but then you wont recharge to 100 percent and will take a little longer. The weaker your VP the weaker your vote. If you give a 100 percent vote but your voting power is only 60 percent the value of the vote will not be as big as when it is at 100 percent voting value.
The vote recharges at 0.833% per hour so over 24 hours it equates to 20 percent.
It is what they would call your "mana bar" if you were to go to a site like steemworld.org or steemd.com . It is basically showing you how much vote power you have left out of 100%.