A posek is someone who decides what is right or wrong according to Jewish law, kind of like a judge. Imagine you're playing a game with your friends and you don't know the rules. You might ask an adult what the rules are so that everyone can play fairly. Well, a posek is like an adult who knows all the rules of Jewish law, and people ask them questions to make sure they are following the rules correctly. The posek can look at different books and talk to other experts to figure out the best answer. Just like how you might ask a grown-up for advice when you have a problem, Jewish people might ask a posek for advice to help them figure out how to live their lives according to Jewish law.