When a fire truck has its siren on and its lights are flashing, it means something important is happening. This is how firefighters tell everyone that they are on their way to help people. Just like how you might use a loud voice to get someone's attention, the siren is a loud sound that tells other cars to be careful and move out of the way.
It's kind of like playing a game where one friend is the 'helper,' and they need to get to someone quickly. So, when you hear the siren, it means you should stop what you're doing and let the fire truck go first. This way, the firefighters can get to where they are needed as fast as possible to help people in trouble!