Definition - What does Overturn mean?
To overturn is to reverse a decision that has been made.
In a legal context, judges commonly overturn court rulings. This is often done by superior courts when an appeal has been made. Once the decision is overturned, the new decision sets the legal precedent and is upheld from then on.
Justipedia explains Overturn
Part of the reason why judges in a superior court (such as the Supreme Court) will overturn court rulings is because they do not want a particular ruling to set a legal precedent for years to come if they do not agree with it.
So, for a example, if a lower court makes a ruling about gun control, and then this issue is appealed to the Supreme Court, the Supreme Court may overturn the lower court's ruling if it finds it to be unconstitutional.