Christians call Jesus their Lord.
The Greek words used in the New Testament for "Lord" are kurios and despotes. The first means "master", as in somebody who has servants. The second can be translated as "ruler", but actually means somebody who has absolute power and authority.
A Christian who knows the Bible will see Jesus as the rightful ruler of the entire cosmos, and will therefore aim to obey Him unquestioningly, unhesitatingly and unreservedly.
Some relevant verses from scripture:
Matt 28:18: Jesus came and spoke to them, saying, “All authority has been given to Me in heaven and on earth."
Luke 6:46: [Jesus said] "Why do you call Me ‘Lord, Lord,’ and not do the things which I say?"
John 13:13: [Jesus said] "You call Me Teacher and Lord, and you say well, for so I am."
John 20:13: Mary said, "They have taken away my Lord".
John 20:28: Thomas said, "My Lord and my God!"
Acts 2:36: "God has made this Jesus, whom you crucified, both Lord and Christ.”
Col 2:6: As you have received Christ Jesus as Lord, so walk in Him.
1 Cor 8:6: There is one God, the Father... and one Lord Jesus Christ, through whom are all things, and through whom we live.
Rev 19:11-16: I saw heaven standing open and there before me was a white horse, whose rider is called Faithful and True. With justice he judges and wages war. His eyes are like blazing fire, and on his head are many crowns... On his robe and on his thigh he has this name written: KING OF KINGS AND LORD OF LORDS.