Okay, kiddo! When you hear a loud sound like a siren, it's like the fire truck is saying, 'Excuse me, I need to get through!' The siren is really loud so that everyone knows the fire truck is coming, and they can move out of the way.
Now, those flashing lights you see? They're super bright and colorful, usually red and white! The fire truck uses them to help people see it, especially if it's dark outside. It's like when you use a flashlight to show where you are. The lights also help to tell everyone to be careful and let the fire truck go fast to help people in trouble.
So, when you hear that loud sound and see those flashing lights, you know that a brave team of firefighters is on their way to help someone!