Answer:
Think of the preamble like the beginning of a story. In this case, it's the start of important rules for a country. Just like when you start a story, you say what it's about. So, in the U.S. Constitution, it begins with "We the People" and says what the rules are for, like being fair, keeping peace, defending the country, making sure people are okay, and giving them freedom.
Other countries also have their own beginnings to their rules. They might use different words, but it's the same idea: saying why the rules are there and what they want to happen.