The Nokia 6 comes with a polarizer layer as part of the display layer stack, which was added to improve the display's sunlight readability, yet still preserving the display\s slim for form factor. The polarizer layer works like a filter and reduces the reflections and glare from the glass. You probably had a chance to use a smartphone with very glossy screens, most phones are. So you know how terrible the visibility gets when trying to see things on the screen outdoors in daylight. The reflections just obscure the visuals and making it very hard to see and read content on the screen. Some phone manufacturers use very bright displays that improve the visibility, and the polarizing layer is another method that helps solve this issue. I don't know how bright the Nokia 6 screen is, but the polarizer layer should definitely be very useful.
In HMD press release, HMD states, and I quote: "..polarizer layer enabling excellent sunlight readability..", so you can be confident that the screen readability will be excellent.