Screen Burns occur in Amoled LEDs when same colors are shown for long duration. In smartphone usually they occur in android navigation menu and in top signal bar. You might have seen lot of apps that are appear in play store but does actually they work.
Apps that you will find online, will burn colors in other part of the display and after as it will burn the same color in whole screen, using them for hours and then you wont able to notice the previous burn. It will actually kill the colors and picture quality of your screen.