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.
Thank you for letting me know this. I thought it was a level up type of thing lol :)