HTTP/3 is the newest version of the HTTP protocol, based on QUIC. This tool checks your headers for the 'Alt-Svc' directive, which indicates the server is ready to accept HTTP/3 connections.
Ready to scan...
It stands for 'Alternative Service'. It tells the browser that the site is accessible via a newer protocol (like h3) on a different port (usually UDP 443).
It is growing rapidly. It runs over UDP instead of TCP to solve Head-of-Line blocking issues.