Voting Power, otherwise known as VP on Steemit, is a measure of how valuable your upvote will be to a specific person's post. This number decreases as you vote on more posts and comments and then increases when you spend time letting your account remain inactive. This is known as recharging your VP.
So how quickly does your VP recharge? Well the short answer is about 20% per day; so if you start the day with 80% VP by that time the next day you will have 100% VP and your voting will be at its maximum power levels.
Remember it is always most efficient to vote with 100% power and as such, dropping below 80% is not recommended if you are an active member of the Steemit platform. This is something that has taken a little getting used to for me because I like to update a whole swath of things, but curating the best content and being picky with my upvotes is likely better for the platform anyway.