HTTP/3 is the new version of the HTTP protocol that uses QUIC instead of TCP at transport level. The specification used in HTTP/3 was previously known as "HTTP-over-QUIC".
HTTP/3 aims to provide connections with greater security, by using TLS 1.3 encryption by default, and higher speed by relying on UDP instead of TCP as in previous versions.
QUIC, the base of the HTTP/3 protocol, has been developed by Google, which also contributed to the development of SPDY (or HTTP/2).
In late 2018 the Chair of the IETF, Mark Nottingham, agreed to rename HTTP-over-QUIC as HTTP/3.
Cloudflare has published a very detailed article about HTTP/3 in which technically describe the new specification. From their own words, conclude: "Like HTTP/2 and TLS 1.3, QUIC is set to deliver a lot of new features designed to improve performance and security of web sites, as well as other Internet-based properties. The IETF working group is currently set to deliver the first version of the QUIC specifications by the end of the year and Cloudflare engineers are already hard at work to provide the benefits of QUIC to all of our customers."