The Droste effect is an endless picture-in-picture—a part of the picture contains the whole picture, which contains the picture, which contains the picture, and on.
Most of us are likely familiar with it from the fun visual effect you often see when sharing your screen on a video call, which contains the video call where you're sharing your screen, on which is the video call where... Always entertaining. Google calls it an infinity mirror.
The name derives from a Dutch brand of cocoa powder, Droste, where the classic advert was a picture of a nurse carrying a tray with a box of the cocoa powder, with the picture of the woman on it carrying the tray with a box of the cocoa...
Articles about the Droste effect often mention the amazing M.C. Escher's Print gallery, a picture without beginning or end.