Definition
Net neutrality is the principle that internet service providers should not discriminate among the data flowing across their networks. They should not block, slow, or charge differently for traffic based on which website it is going to, which application is generating it, or which company owns it. The principle has been variously codified, weakened, and re-strengthened in different jurisdictions over the past two decades. In the EU it is law: the Open Internet Regulation (EU) 2015/2120 bars ISPs from blocking or slowing specific lawful apps and services, apart from narrow, transparent traffic management.
Why VPNs Are Relevant
Without net neutrality, an ISP can throttle competing video services to favour its own, charge extra for 'unlimited' use of specific apps, or block lawful content it disapproves of. A VPN encrypts traffic so that the ISP cannot identify which service is in use and therefore cannot apply differential treatment based on protocol. In jurisdictions without strong net neutrality protections, a VPN is one of the few practical tools individuals have to enforce equal treatment of their traffic.
Limits
A VPN can defeat protocol-based and destination-based throttling but not blanket throttling that applies to all encrypted traffic. Some ISPs in heavily regulated markets throttle all VPN traffic equally, on the theory that any VPN is suspicious. In those environments, only obfuscated VPN protocols that hide the fact that a VPN is in use can restore full speeds.