The private one is revealed after the competition is over. Only the private leaderboard counts. To get an idea of how well your model performs, you get scores f

The private one is revealed after the competition is over. Only the private leaderboard counts. To get an idea of how well your model performs, you get scores for, let's say 30% of the holdout data, on the public leaderboard. You can also use local cross-validation to gauge model performance. There is a limit on the number of submissions. Usually 5 or less a day. Also precision of scores is visibly limited, but for final ranking all decimals count.