Hi Nicky
Hope below helps. Got it from the guidelines
This is like an "energy bar" in a computer game that gets depleted every time you vote. You start off with 100% and then every time you vote your power and hence the value of your vote goes down. This makes the value of the payout from your vote go down. Think of it as a control measure to stop you up-voting everything you see. Currently the only way I know of seeing voting power on the desktop is to use Steemd (https://steemd.com/@youraccount)- I believe the mobile client (eSteem) does show it though.