Ghost image or screen/image burn-in are names given to a permanent discoloration of your smartphone’s screen caused by irregular pixel usage. The prolonged use of static images can create a permanent shadow or ghost of that image on the screen. This problem is more common than you think, and happens most often on AMOLED screens (although LCD displays aren’t completely free of this bug). Fortunately, there is a solution to restore the image quality of your device.
The screen ghost happens when phosphor compounds that emit light to produce images lose their intensity with prolonged use. Moreover, the irregular use can "burn" an image onto the screen which will be visible all the time.
Many apps that are available in the Play Store promise to reduce or even stop the problem. You'll want an app that's compatible with your display type.
For LCD screens: there's a dedicated app, LCD Burn-in Wiper. However, this tool is not suitable for OLED or AMOLED displays, such as those found on Samsung Galaxy devices. For that, you'll need a different app.
The concept is simple: a sequence of primary colors is displayed on your device, restoring the "burnt" pixels. In fact, this was the original function of computer screen-savers: one dynamic image that appears when the screen is idle to makes the pixels "exercise" and ensure that the same area of display doesn’t remain constantly illuminated.
Did your smartphone ever had this problem? Did you use another app to fix it?