Using God's name in vain is saying "God" or "Jesus" or "Christ", etc. without meaning to call on God or talk about God with respect.
People of this world use God's name in vain frequently almost like a curse word. For example, something bad happens and they say "[...]". This is using God's name in vain.
Sometimes, Christians out of ignorance do this too, because maybe they hear other people do this.
So they use God's name in a sentence like a filler word or an exclamation. This is also using God's name in vain.
We as God's children should only use God's name when we are actually talking about Him (like in this post for example).
And we should use God's name when we call upon Him in prayer or when we talk to Him.
God's name should always be used in a way of respect and reverence for God, because He is the great King. His name should never be used as a filler word, proclamation of surprise, or curse word.
So whenever we use God's name, we need to be careful. Do we really mean to call upon Him at that moment?
If you've used God's name in vain before, ask for God's forgiveness and He will forgive you.
In the future, refrain from using His name in vain and ask God to help you remember to always use His name in reverence and meaningfully.