Okay kiddo, have you ever stayed up really late and watched television? Sometimes, when it gets very late, the TV station will put on something called a "nightscreen" instead of regular programs.
The nightscreen is like a blank screen with some pretty pictures, colors, shapes, and sometimes music or sound effects. This helps people who are still awake to have something nice to look at or listen to while they wind down for bed.
It's kind of like a nightlight for TV - it keeps the TV on so we don't feel scared in the dark, but it's not as bright or loud as regular TV shows that might keep us awake longer.
Some people even like to fall asleep to the nightscreen because it's calm and soothing. And when we wake up the next day, the regular TV shows will be back on!