Very often in the Gospels people address Jesus as "Lord". This title testifies to the respect and trust of those who approach him for help and healing. At the prompting of the Holy Spirit, "Lord" expresses the recognition of the divine mystery of Jesus. In the encounter with the risen Jesus, this title becomes adoration: "My Lord and my God!" It thus takes on a connotation of love and affection that remains proper to the Christian tradition: "It is the Lord!"
Source: Catechism of the Catholic Church (Vatican.va)