This is the target of the "fixer video." A stuck pixel is a pixel that remains lit as a specific color (usually red, green, or blue) and does not change when the image on the screen moves. This happens because the liquid crystal in that specific cell is "stuck" in one position or the transistor is sending a constant signal. Because the pixel is still receiving power, it is often possible to unstick it through rapid color cycling—which is exactly what a fixer video does.
Once you fix your screen, protect it: