Leadership is one of the most important aspects of the game. It determines how many troops of each type you get to send into battle. Leadership increases per level are listed on Levelup Rewards page.
The starting Special unit, Sea Wolf, has a Leadership stat of 100. If you personally happen to have a Leadership stat of 500, you would be able to send out 5 of the Sea Wolf troops in your Sea Wolf unit stack. The first "swarm" unit you acquire, the lowly Peasant, has a Leadership score of only 10. This means that your Leadership stat of 500 will give you 50 Peasants! While a low Leadership score generally means a low everything else score for balancing purposes, it's always a good idea to consider a unit's Leadership when determining which of your units is the strongest, just don't forget to take everything else into consideration too.