What the burn-in & image retention test shows
Burn-in is permanent: pixels that have displayed the same bright content for thousands of hours age faster than their neighbours and never fully recover. It shows as a faint permanent ghost of a taskbar, a channel logo, or a game HUD.
Image retention is temporary and looks identical at first. It fades over minutes or hours of varied content. The only way to tell them apart is to wait — which is why the honest answer to "is this burn-in?" is almost always "run varied content for a day and look again".
Telling burn-in from image retention
Step through the colours slowly and look for shapes: a rectangle where a taskbar sits, a logo outline, the frame of a window you keep in the same place. Grey shows it best; red and green often show it more strongly than blue.
If you find something, run varied content for a few hours and test again. Retention will have faded noticeably. Burn-in will be exactly where it was.
What this test cannot tell you
Nothing here can remove burn-in. Pixel-refresh routines built into OLED televisions can reduce retention, but genuine burn-in is physical ageing of the emitters and there is no software fix — anything claiming otherwise is selling something.