A case of first impression is a situation where a judge has to make a decision that has never been made before. It's like the judge is the first person in the whole world to solve a really tricky puzzle!
Sometimes, people do things that are new and different, or there are new rules or laws that haven't been used before. When that happens, the judge has to figure out how to apply the rules and laws to the situation. They have to use their best judgment based on what they know and what they think is fair.
It's kind of like when you play a new game for the first time. You don't know all the rules yet, so you have to figure them out as you go along. Once you know the rules, you can play the game better next time. Similarly, once a judge has made a decision in a case of first impression, other judges can use that decision to help them in similar cases in the future.