Okay, little buddy! When you hear a loud sound like a siren and see flashing lights, it means a fire truck is coming. This is because firefighters are going to help someone who needs them.
Just like when you play with your toys and want everyone to share, when the fire truck is on the road, it wants other cars to move out of the way. That helps the firefighters get to the place they need to go very quickly and safely.
So, when you see or hear that siren, it's a sign to let them pass because they are doing an important job!