http://www.eu4wiki.com/Discipline
actually decribes it pretty quickly without having to dig through.
It explains why discipline is so awesome you can only take it to numbers like 42.5 when you really really focus on it. If brandenburg walks around with 35% discipline they do 35% more health damage, 70% more morale damage, take 35% less health damage, and take 70% less morale damage.
So just like above 10 discipline is worth 10 tactics and 10 combat ability but it's not easy to get tactics up past your enemies.
The last time I played hindustan I focused heavily on discipline and would easily reach 37.5 at times. When I stacked that on top of tactics from technology I could crush large groups of enemies with small groups of units.