The third of the Ten Commandments, found in Exodus 20:7, tells us how to worship the right God the right way, even down to the use and mention of His name: "You shall not take the name of the Lord your God in vain, for the Lord will not hold him guiltless who takes His name in vain."
What does that mean for us today? To use God's name in vain is to empty it of its value and meaning, or to use it in a way that's not true to His character, reputation, or authority. Basically, the name of God should not be uttered, written, or sung in any kind of empty, frivolous, or insincere way.
I think some people have trouble with this commandment—"What's the big deal? They're just words"—but it's actually very important. In fact, everything we say is important, because, as Jesus put it, "Out of the abundance of the heart [the] mouth speaks" (Luke 6:45; see also Matthew 12:36-37). In other words, your mouth is a barometer of your heart.
Now, even if you don't say things like "Oh my God" or "Jesus Christ" in a defaming way, did you know there are other ways you can take the Lord's name in vain? For example, using His name lightly. I cringe when people tell me, "God told me such-and-such." I know God can speak, but there are a lot of times when I'm not even sure God spoke to me. What if it was just that late-night burrito with onions? Then you just took the name of God in vain.
You can also take God's name in vain through profanity, where you use His name in a coarse manner, like asking Him to damn something or someone. Invoking God to doom someone to destruction does not represent His character, reputation, or authority. It's not His heart. He isn't about condemning people; He's about saving people (see John 3:17; 2 Peter 3:9).
Another way you can take God's name in vain is through hypocrisy. This is when people take on the name of God, saying, "I'm a Christian," but it's a profession without a practice, words without a walk. These are people whose actions cancel out their words (see Matthew 7:21-23).
When Jesus taught us to pray, He said, "In this manner, therefore, pray: Our Father in heaven, hallowed be Your name" (Matthew 6:9). In other words, the first concern you should have is for the character, reputation, and authority of God to be highly esteemed. You've got His name. What are you doing with it?