Sometimes a lawsuit is filed against the wrong person. The most common example is when you get sued because you have the same name or a similar name as the person who was supposed to be sued.
You should respond to the lawsuit so that the judge is informed of the mistake. If you do not appear in court and contest the case properly, a judgment will be entered against you even though that sounds unfair.
The judgment is the court's decision in the case. And even though you were the wrong defendant, you will have to pay the judgment if you do not fight the lawsuit.