In the Sermon on the Mount, Jesus explained the second commandment: "You have heard that it was said to the men of old, 'You shall not swear falsely, but shall perform to the Lord what you have sworn.' But I say to you, Do not swear at all.... Let what you say be simply 'Yes' or 'No'; anything more than this comes from the evil one." Jesus teaches that every oath involves a reference to God and that God's presence and his truth must be honored in all speech. Discretion in calling upon God is allied with a respectful awareness of his presence, which all our assertions either witness to or mock.
Source: Catechism of the Catholic Church (Vatican.va)