Resolutions are nothing but just some horizontal and vertical pixels. But these pixels make a huge difference in the quality. The more pixels, the better the clarity and sharpness of the image. For example, the 4K UHD offers about four times more pixel count than the FHD. so at the same dimensions you are getting a large angle of coverage. That too without the image getting any extra blurry. And the pixels are also the reason for resolutions being distinguished. How many pixels a resolution can cover in a respective area decides which resolution is different.