Let's compare the Internet to a highway system, since that's the analogy politicians like. The Internet is all private roads. You pay your ISP for your road so that any traffic serving you can come across in either direction. I pay my ISP for any traffic serving me to come across in either direction.
Net neutrality says that if you and I send traffic to one another, your ISP can't charge me for my packets (trucks) carrying data (cargo) to you because you've already paid for use of the road. They also can't charge you more for a truck entering your road from my road than from their own other road, or from Google's road. It's all the same toll (or all included if you have an unlimited traffic plan) and you pay the same for your leg of the trip no matter how the truck got to your road.
Anti-net-neutrality folks think the government should grant the folks running the private toll roads we rent the right to waive tolls on those roads if they have a business relationship with one of the parties. They think the toll road operators should be able to charge you more rent or higher tolls if you do business with someone whose cargo came from a competitor's road to get to your section of road. At the same time, they say it's fine that the government itself has severely limited your choice in local road providers giving you no power to negotiate or find a suitable competitive road provider.
Then anti-neutrality folks call that an open, free market. Free for whom? Open for whom? Your state tells you who can install a road without a fair bidding process, that whoever installed the wholesale road doesn't need to allow a competitive resale market for retail use of the road, that you can't individually negotiate terms with the road operator, and that you can't build your own road to replace it. Now the federal government wants to tell you that the road operator under those terms set by your state can shut out cargo from its roads or charge huge surcharges based on who you bought the cargo from. What's free and open about that?