The function in a 4K TV that increases the incoming resolution to render on the 4K screen. Because a 4K TV has four times as many pixels on the screen as a 2K 1080p TV, it analyzes the incoming signal and "fills in the blanks" for non-4K content. As of 2019, there is a limited amount of 4K material available.
Blu-ray players and A/V receivers may also do 4K upscaling. However, in most cases, the TV does the best job because it is dealing with its own internal circuitry and architecture. If the user allows the Blu-ray player to upscale to 4K, the A/V receiver must be able to pass the signal through to the TV without any processing. See 4K pass-through
and 4K TV