"Do not take the name of the Lord in vain," prohibits the blasphemous use of the Lord's name. Over the centuries, this commandment has been understood in different ways. For example, it was taken to mean the utterance of the name of God in any instance was blasphemous. Today, it is mainly taken to mean that the name of the Lord shall not be used to bear false witness, or to curse (e.g., "God damn it!").
Jesus, in his Sermon on the Mount, talked about avoiding the use of the Lords name in vain, by being honest, and not inviting a situation in which one might be compelled to invoke the Lord’s name in order to earn credibility.
The Catholic Church has a slightly different approach, and instructs that the Lord’s name should only be used to glorify, bless, and praise him.