Generally speaking, a lord is someone with authority, control, or power over others. In Jesus’ day the word lord was often used as a title of respect toward earthly authorities. When the leper called Jesus “Lord” in Matthew 8:2, he was showing Jesus respect as a healer and teacher.
However, after the resurrection, the title “Lord,” as applied to Jesus, became much more than a title of honor or respect. Saying, “Jesus is Lord,” became a way of declaring Jesus’ deity. It began with Thomas’ exclamation, “My Lord and my God!” (John 20:28). From then on, the apostles’ message was that Jesus is Lord, meaning “Jesus is God.” Peter’s sermon on the Day of Pentecost contained that theme: “God hath made that same Jesus, whom ye have crucified, both Lord and Christ” (Acts 2:36). Later, in Cornelius’ house, Peter declared that Jesus is “Lord of all” (Acts 10:36).
In saying, “Jesus is Lord,” we commit ourselves to obey Him. Jesus asked, “Why call ye Me, Lord, Lord, and do not the things which I say?” (Luke 6:46). An acknowledgement of Jesus’ lordship is logically accompanied by a submission to Jesus’ authority. If Jesus is Lord, then He owns us; He has the right to tell us what to do.
A person who says, “Jesus is Lord,” with a full understanding of what that means (Jesus is God and has supreme authority over all things) has been divinely enlightened: “No man can say that Jesus is the Lord, but by the Holy Ghost” (1 Corinthians 12:3).
—Condensed from GotQuestions.org