I want to know more about this.. it’s is a very serious issue or not?

That message means that the display was replaced with a display that does not meet Apple’s specifications, or the display has failed if it is an Apple display or was replaced by Apple. If it is working you can continue using the phone without any problem, but some features may not work, such as True-Tone and automatic brightness. That warning will not go away, however.

Until recently genuine Apple displays were only available to Apple Stores and Apple Authorized Service Providers. Apple will be selling genuine displays on the open market in the near future.

It's just going to nag you about it. And even with some of the newer iPhones there have been reports of installation of salvaged parts leading to display and battery messages, even when they were OEM. There could be the possibility of a glitch even with an completely original Apple device. If it's an original part, then only Apple or an Apple Authorized Service Provider might have the equipment needed to clear such a message. I don't believe they can do anything about it if it is an aftermarket part. But you can continue to use it.