HTTP2 allowed multiple streams over one TCP stream, but that kinda made HoL blocking worse, because in the same scenario HTTP 1.1 would have just opened multiple TCP streams. QUIC, as GP said, basically gives you a VPN connection to the server. Open 100 streams for reliable downloads and uploads, send datagrams for a Quake match, all over one UDP 'connection' that can also reconnect even if you change IPs.
HTTP2 allowed multiple streams over one TCP stream, but that kinda made HoL blocking worse, because in the same scenario HTTP 1.1 would have just opened multiple TCP streams. QUIC, as GP said, basically gives you a VPN connection to the server. Open 100 streams for reliable downloads and uploads, send datagrams for a Quake match, all over one UDP 'connection' that can also reconnect even if you change IPs.