All of the characters in the game have ratings based on how skilled they are at leading infantry, calvary and archers. Those are the numbers on the bottom of the screen during assignments. You want to select the people who give the biggest boosts to the groups that you have in your party.

If you have all infantry, you don't care about archer boosts, for example.

Does it give any numeric stat boost or just boost their AI leadership or something?

It gives whatever boost it says on the screen. To every troop that's classified in the same group.

I think the attack% is a damage bonus and the defense% is damage reduction bonus. The bonuses can get pretty high with skilled leaders, like 30-40% or more