The leaderboard shows the top 5 leading teams in each category. It is updated every hour. The leaderboard will only show one entry per team, this means the even if a single team holds the top two scores for FScore, only the highest entry for that team will be displayed.
The leaderboards are for best FScore (also called F1 Score), best Precision (with Recall >= .9), best Recall (with Precision >= .9) and best FScore on initial submission. A good description of Precision, Recall and FScore can be found on Wikipedia.