Anyone can vote for a server once every 24 hours. Voting pushes a server up the rankings, helping it get discovered by more players.
To vote, open a server's page and hit the Vote button. If you're logged in, you can also leave a review after voting. Votes reset at the start of each month, so the monthly leaderboard is always a fresh competition.
Server owners can reward you in-game for voting if they've set up Votifier (see below).