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.