Appearing over 700 times, kyrios is a paramount christological title. In everyday Greek: "sir" or "master." In the Septuagint, it translates both Adonai and the divine name YHWH (H3068). Applied to Jesus, it carries the full weight of the divine name.
The earliest Christian confession was "Jesus is kyrios" (Romans 10:9). In the Roman Empire, this denied Caesar's ultimate authority. Philippians 2:9–11 quotes Isaiah 45:23 (where YHWH speaks) and applies it to Jesus — every knee will bow and confess that Jesus Christ is kyrios.
The use of kyrios for both YHWH and Jesus is one of the strongest NT arguments for Christ's deity. Joel 2:32 says everyone who calls on kyrios (YHWH) will be saved — Paul applies this directly to Jesus (Romans 10:13). Jesus IS the YHWH of the Old Testament made flesh.