windowscentral[.]com/software-apps/windows-11/how-to-use-the-hdr-calibration-app-on-windows-11
This is Very Much a mileage will vary sort of thing.
In the US most TVs on sale include HDR [High Dynamic Range], & the same goes for many hi-rez PC monitors -- an easy, basic explanation of HDR is here: coolblue[.]nl/en/advice/hdr-monitor.html
Marketing departments feel that adding HDR to their product's label [& specs] will sell more TVs & monitors. The official HDR specs, which offer all sorts of leeway, make adding HDR to a display cheap. That also means that HDR label can be almost meaningless, or it may matter quite a lot, depending on which features of the specs a particular display includes. To further complicate matters, PCs and their GPUs, software, & games aren't very good at HDR, at all.
While it's pretty much broken in Win10, Microsoft is trying to make HDR mean something at least somewhat positive in Win11 -- the colors may or may not be terribly accurate, but hopefully some people will think it looks a little better, and that includes playing games that support HDR.
The calibration app is used to create a new calibration table -- Windows uses a calibration table to map the colors of everything you see on-screen to monitor-specific values for a more accurate representation of what they should look like. Normally the calibration table used is a more generic table created by the display manufacturer, or you can buy a calibration device that measures a display's color output, then creates a new calibration table [profile] based on that individual display. What the HDR calibration app does is determine the darkest & brightest level the display is capable of, then stretches a generic table to fit both extremes, while Win11 itself tries to enhance the color for HDR.
The results may be better, or worse, depending on what you want &/or like to see, the capabilities of your display, the software you use for watching HDR video, &/or the games you play. The good news is that it's not a lot of work to turn on HDR, install & run the calibration app from Microsoft's store, and give it a go. The way you turn HDR on/off in games varies, and for games there's also Win11's Auto HDR, that you turn on/off in Settings where you turn HDR itself on/off [ pcworld[.]com/article/546269/auto-hdr-deep-dive-how-and-why-to-use-windows-11s-eye-catching-game-feature.html ] If you don't like the results you can turn HDR off & restore your previous calibration table in Windows Color Management.