What it improves
Built on QUIC over UDP, it removes head-of-line blocking at the transport layer and recovers faster from packet loss — which is where mobile networks spend much of their time.
Who notices
Users on congested mobile connections see the largest improvement. On a good fixed line the difference is often marginal.
How to enable it
Usually a server or CDN toggle rather than an application change. Clients that cannot negotiate it fall back to HTTP/2 automatically.