Alright kiddo, have you ever noticed how sometimes the world around us is quiet and sometimes it's loud? Well, sometimes we measure how loud it is with something called a noise level.
During the day, when we're all up and about, walking around and talking to each other, there's usually a lot of noise. The noise level is higher during the day because there are more people around making sounds with their voices, cars driving on the roads and machines working.
As the day starts to come to an end and the sun starts to set, things start to calm down a bit. People start to go home and the streets start to become quieter. The noise level starts to decrease as the day turns into evening.
Finally, when it's completely dark out and everyone is asleep or inside, the noise level is usually at its lowest. There aren't as many people making noise, and even the animals are usually quiet. This is why we call this time of day night.
So, in summary, during the day there's a lot of noise, during the evening it's quieter, and during the night it's the quietest.