Net neutrality is about providing to users the same quality of service (ie. mainly the bandwidth capacity you paid for) to access all services on the internet. In other words, an internet service provider (ISP) should not discriminate one traffic over another. Since it does concern the last mile infrastructure it depends on the local regulations which means obviously that it might be different from one country to the other.
To keep growing and monetize their infrastructure ISP needs to find new revenue flow and one outcome could be to head towads tiered-internet. ISP might be tempted to make pay selected services for a better quality of service (ie. insure bandwidth) on their network, hence granting for the end user the best possible experience for this service. While getting the best service possible without spending a penny sounds good for the end users (ie. us), it has a drawback: it gives priority on the network to the companies that have the most money, and since start-ups are not part of those it would basically kill innovation.