"You shall not misuse the name of the LORD your God, for the LORD will not hold anyone guiltless who misuses his name." Exodus 20.7
What's the problem? People do it all the time. Even religious people... Yet there is a prohibition on using God's name thoughtlessly up there at Number 3 in the 10 Commandments. Why is it such a big deal?
Apart from the obvious (disrespect towards the Lord of the Universe, Author of Life, Saviour of the World, &c), misusing anyone's name undermines our relationship with them. When it comes to God, that threatens our very existence and our destiny. Because, if we are not in a correct relationship with God, then we do not belong to him and, if we do not belong to God, we are fodder for the Devil.
The Israelites of old took this commandment so seriously that they never addressed God by name, using only a circumlocution, such as "the Lord" or simply "God". This is why there exists uncertainty over the correct pronunciation of the "tetragrammaton", the four consonants (written Hebrew does not necessarily include vowels) which express God's actual name. It can be rendered "Jehovah", though scholars prefer "Yahweh".
Taking the Lord's name in vain amounts to much more than mispronunciation, of course. The injunction covers everything from referring to God disrespectfully to using such references, be they his actual name or one of its periphrases, as a swear word. Just don't do it, is the message. No one wants to be judged guilty by the Ultimate Lawgivier!