What do we mean by calling Jesus Lord?

Paul began his letter by saying, "...separated unto the gospel of God, which ..."
We understand the word "gospel" in two different ways.
First of all, as a text type, gospel was created by Mark to tell the story of Jesus, his words and deeds. It was based on some historical events and written in such a way as to make its readers believe more in Jesus. There can be found a lot of editorial efforts and therefore, we cannot treat it as a kind of "objective", historical biography of Jesus. The Evangelists had put in a lot of their interpretation of the Jesus events. Their narratives were coloured in a particular way.
Secondly, "gospel" literally means "good news". Therefore, the underlying messages of the gospel are good news to the readers. Then, what is so good in particular about the news? Simply put, through the life of Jesus, God reconciles with us sinners. Despite our disobedience, God does not give us up. He sent His Son, Jesus Christ, to deliver us from the bondage of sins. We were created in the image of God (Genesis 1:26). However, we do not live up to the standard, to show off the image of God. Jesus came to us to demonstrate to us what an image of God is capable of. Paul was chosen to be a servant of Jesus Christ, to bring the gospel to the Romans.

Concerning his Son Jesus Christ our Lord, which was made of the seed of David according to the flesh; (Romans 1:3)
Christians of the first century called Jesus Lord. What did they mean?
The first Christians were Jews. They kept the Ten Commandments one of which forbids calling God's name directly. So, when they came to the tetragrammaton YHWH (pronounced as Yahweh), the 4 alphabets that represent God's name, they would instead pronounce "Adonai", which is "my Lord" in English. Therefore, for the first Christians who were Jews, calling Jesus Lord was equivalent to calling Jesus God. Jesus is their God.
What did it  mean to be the God of the Jews?
The Torah emphasizes many times that God has entered into a covenant with them. They are the Chosen People of Yahweh. They are His inheritance. Yahweh has delivered them from a house of slavery into Canaan, the Promised Land. When they follow the Torah to lead their lives, God would show them mercy. Otherwise, God would punish them.
So, unlike deities of other peoples or tribes which could be bribed and made use of to produce fortunes for their worshipers, Yahweh acts in a totally different way. Yahweh wants to be personal. Yahweh wants to enter into long term relations with His people.
Therefore, to be the Lord/God of the first Christians, Jesus must enter into a covenant with them. Moreover, Jesus must deliver them from the slavery of sins and promise them a piece of land etc.

What about modern Christians? When they call Jesus Lord, what do they mean?
Unlike their first century counterparts, modern Christians do not have earthly lords ruling over them. They don't have any example as reference. The word 'Lord' is completely void of meaning for them. Moreover, they are technologically more powerful. They are more assertive and have taken into their own hands a greater portion of their lives. I do not mean they will necessarily be more arrogant. But they will not hesitate to make their voice heard. They will object and complain any "unreasonable" policy of the Church. They want to take over the control of their lives instead of surrendering them to Jesus. Therefore, it is hard to imagine modern Christians submitting to an individual, ho Kyrios. I may even speculate that "The Lord" is merely a title Christians give to Jesus without much thought about its meanings and significance in their life. The title means nothing to them.

I also call Jesus Lord. In most of my concluding prayers, I invoke the Lord instead of calling out for Jesus. Rationally, I know that Jesus can have full control over my life and my destiny. In reality, I seldom let go and always jump at the first opportunity to take control over my own life. I am still a long way behind the stage when I can heartily pray that God's will be done.

Dear Lord, I sincerely offer up my life for You to use. Make me Your handy instrument. Amen.