What does "taking the Lord's name in vain" mean?

This is the third commandment.

“You shall not misuse the name of the LORD your God, for the LORD will not hold anyone guiltless who misuses His name.” This is a command against taking the name of the Lord in vain. We are not to treat God’s name lightly. We are to show reverence to God by only mentioning Him in respectful and honoring ways.

It's the exact opposite of "Hallowed be thy name." It's using the name of God in ways it was never intended, therefore its taking it and using it for nothing. Hallowed just means to render as sacred. Whenever His name is used, reverence, honor and awe should be related to it.

It means that some one was trying desperately to come up with a set of rules that they could pretend that they got from God.

Apparently they assumed that God was as insecure as they were.

Love and blessings Don

Technically, it refers to taking an oath with G-d's name falsely. It also applies to any usage of G-d's name in profanely.

It means if you pray for something and your mind is somewhere else or you dont pray with you whole heart