Specifically, a thumbnail refers to a small image that summarizes or exemplifies a larger piece of content, such as a video. On YouTube and other video-hosting websites, a thumbnail is displayed as a small, still image. This is the image that you see before you click on a video, making it your first impression.
When you upload your video to a site like YouTube, you have the option to either choose and upload your own thumbnail or allow the platform to auto-select a still image from your video. In general, you’ll want to at least select your own video still to use as a thumbnail. Ideally, though, you’ll be making your own from scratch and uploading it.