Also called preview image, cover image
A thumbnail is the small preview image standing in for a video or link, and it decides most of whether the thing behind it is opened.
A thumbnail is competing in a grid at a size where detail disappears. What survives is one subject, high contrast, and at most a few words — everything else becomes texture.
It also has to work as a pair with the title. The two should not repeat each other: the image showing what the title already says wastes half the space, whereas an image raising the question the title answers earns the click.
Consistency across thumbnails is what makes a back catalogue recognisable in a feed, which matters more than any single one performing well.
In practice
A face with one clear expression and three large words reads at 120 pixels wide; a screenshot of a spreadsheet does not.
Common mistake
Designing at full size. Thumbnails must be judged at the size they will actually appear, which is roughly a thumbnail.
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