"You shall not bear false witness against your neighbor.
Very loosely, the ninth commandment is to never lie.
More specifically, it is about being truthful in an agreement. For example, that you will do the work which you agreed to do for the payment that was agreed upon by your employer. Or more commonly seen these days is in keeping an oath such as a citizenship oath or in a court of law when "swearing in" to testify.
The command goes deeper than many people think. Contained within this is the knowledge that we should be honouring and respecting God with our words. How we deal with people will reflect on God, and we must aim to represent his good character in ourselves. We could use this command to tell the truth but still deceive. God's people are to avoid breaking more than just the law, but avoid breaking the 'spirit' of the law.