league of legends how armor pen works?

Answer 1

Armor Penetration and Armor Reduction are the statistics of a Champion which allows them to counter some or all of a target's armor.

The target's armor is reduced by the amount of armor penetration.

source:http://leagueoflegends.wikia.com/wiki/Armor_Penetration

Answer 2

For every point of armor penetration you have, it negates a point of armor on the target.

 

Example:

 

You have 20 armor pen. and your opponent has 100 armor. when you attack with a physical attack, it will be as if your opponent only had 80 armor

Answer 3

To put it simply, it penetrates armor, allowing you to deal more damage. 20 armor pen will negate 20 armor of your opponent, so your physical attacks will deal damage as if the enemy has 20 less armor. Quite simple - no negative armour though!