We found the best ever explanation of net neutrality here;
Think about data like you think about water. A certain sized pipe can only deliver water at a certain maximum speed, and it takes electricity and pumps and infrastructure to do that.
Then think about the kid in his basement running multiple torrent servers as the guy who wants to fill his swimming pool with water every few hours. It takes electricity and pumps and infrastructure to do that.
Then think about grandma, who can save $ 200 a year off her phone bill by using Skype or MagicJack instead of the government regulated phone company.
Net neutrality makes sure that all the kids filling their swimming pools with data prevent Grandma from making a phone call.