Full definition
QUIC is Google's replacement for TCP underneath HTTP. HTTP/3 runs over QUIC instead of TCP, getting faster connection setup (no TCP+TLS handshake serialization), better multiplexing, and graceful handling of network changes (you can move from Wi-Fi to cellular without dropping connections).
For proxy users this matters in two ways: (1) some destinations now require HTTP/3 to look fully modern, since real browsers prefer it when available, and (2) traditional HTTP proxies don't support QUIC because they expect TCP. SOCKS5 with UDP support is the workaround.
Major providers are starting to add HTTP/3 support: Bright Data and Oxylabs both ship UDP-capable SOCKS5 endpoints. If you're scraping a site that fingerprints clients on whether they speak HTTP/3, you'll want this.