You're probably thinking of the third of the Ten Commandments that God gave to His people. This commandment says, "Thou shalt not take the name of the Lord thy God in vain; for the Lord will not hold him guiltless that taketh his name in vain" (Exodus 20:7, KJV). This is simply an old-fashioned way of saying we aren't to misuse or abuse the name of God.
What does this mean? It can mean simply using God's name in a casual or meaningless way in our ordinary speech. (This applies to any name or title for God, and also for Jesus, the divine Son of God.) But when we use God's name this way, it means we don't take Him seriously - and that is wrong. Our speech is a reflection of our hearts - and when we treat God lightly in our speech, it indicates He isn't really important to us.
Sometimes, however, people misuse God's name in a more deliberate way - for example, calling on God to condemn someone we hate. But that also is wrong, for only God is their judge. Here again, our speech is a reflection of our hearts - and it shows that our hearts aren't right with God.
The Bible says, "Do not let any unwholesome talk come out of your mouths, but only what is helpful for building others up" (Ephesians 4:29). Don't be influenced in your speech by those around you, but yield your tongue - and your life - to Christ.