Net Neutrality is specifically the idea that delivery of bytes of data over internet connections amounts to a global public commons that is a shared resource for everyone. Just as we make laws to prevent someone from putting up an enclosure fence around public meadows, we also have the idea of net neutrality so nobody puts up a virtual enclosure ("high priority lanes") around the internet. i.e. everybody pays the same rate for traffic delivery regardless of the origin or content of the traffic.
you're right that this was an inevitable power play that would be made by monopolist rent-seeking corporations as soon as they could get away with it. the importance of strong regulations on this issue cannot be emphasized enough.