Screen burn-in happens when a particular image remains on the screen for very long periods of time without moving, causing something like a ghost image to remain on your screen when the image is gone. It used to be very common with plasma screen TVs, but technology has improved over time to reduce the possibility of burn-in.
Always keep iOS up to date - Apple uses software technology in the iPhone X to help combat potential burn-in effects. Whenever there is a new update, you should install it. Not only will you get new features, but there might also be a specific display screen issue that you'll be able to avoid.
The first step is to turn off iPhone for a while. If your iPhone has a slight bit of image retention, you might be able to fix it by powering off your iPhone for 15 minutes.
Next, turn it back on and use it normally, even if you still see the image ghost. It's possible that the retention will disappear as your iPhone display is used for all of its normal activities.
If it doesn’t disappear on their own, you may have to replace the screen. If it's true burn-in and not just a slight bit of retention, there's not a lot you can do about it. If your iPhone X is new, its probably something you'll want to replace.