The thumbnail is the still image that represents a video before it plays: the first impression that decides whether someone opens it. It can be an automatic frame from the video itself or an image uploaded by whoever publishes it, designed to stand out in a list of results or recommendations.
In ōrbita
On YouTube you can upload your own thumbnail: a JPEG or PNG image of up to 2 MB, cropped to 16:9 (1280×720 recommended). YouTube only accepts custom thumbnails on phone-verified channels; if it rejects yours, the video is still published, without a thumbnail, and ōrbita lets you know. On Shorts you cannot set a thumbnail from ōrbita: YouTube always uses a frame of the video itself. Details in the YouTube guide.
Also known as: video cover, cover image, video poster.