8K is a higher resolution than 4K, and nothing more. The resolution of a regular 1080p display is 1920 x 1080 pixels. 4K displays double these figures to 3840 x 2160, quadrupling the number of pixels. 8K doubles these figures again to 7680 x 4320. That's four times as many pixels as 4K, which means it's 16 times more than a 1080p TV.
To better understand, try looking closely at your TV. Try to find a single pixel (not the individual red, green and blue lights; these are sub-pixels, meaning you are too close). If you're looking at a 4K screen, imagine that four pixels take the place of one pixel. If you're looking at a 1080p display, imagine that this one pixel contains a grid of sixteen pixels, four by four. That is 8K. That is a much sharper image than 4K and much, much sharper than 1080p.
While 8K looks sharper on paper, it remains to be seen whether 8K will look much better to your eyes. As with all TVs and monitors, how clear and sharp each pixel is depends on the size of the screen and the distance from the viewer.