In the Player's Poll, every player has the chance to vote on the best performances by their team mates. You can only vote for players in a game in which you have played, and you cannot vote for yourself. You get one vote per game and you can award 1, 2 or 3 points.
To vote you must be logged in - click on 'Matches' and you should see a link for voting beside each match for which you are eligible.