Hey kiddo, have you ever played with the same toy for a really long time and then noticed that the picture or letters on the toy don't look as bright or clear as they used to? That's kind of like what happens with screen burn-in.
You know how your family has a big TV or computer screen? Well, when people use those screens a lot, sometimes the picture or words can get burned into the screen. That means that even when you turn off the TV, you can still see a little ghost of the picture or words that were on the screen before.
It happens because the pixels on the screen get used so much in one spot that they can't change colors as well as they used to. So, even when the picture changes or you turn off the screen, that spot still looks a little bit like what was on the screen before.
It's kind of like when you draw on a piece of paper with a marker for a really long time in one spot. The ink gets so thick that you can still see a faint image of what you drew, even if you try to erase it.
That's why it's important to take breaks from using screens and turn them off when you're not using them. That way, the pixels can rest and you won't have to worry about burn-in.