@goofsqueak@lemdro.id is correct, it's an AI generated thumbnail option that youtube suggests to content creators as it is "calculated" to get the most clicks, but i think it has been backfiring. A channel i watch a lot started to use it and everyone pushed back, so they stopped.
The main thing to look out for is chunky yellow letters. Often a bit of crinkling on the edges as so - more so if it's "appropriate", e.g a biking video (pictured above). Always to the side, usually the entire left half of the screen.
When you go to make a video it suggests the thumbnail to you based on your video content, or you can upload one and it will suggest changing it. It's optional, but lots of creators "fall" for it because it saves them work and insists it gets more clicks