A video thumbnail is a still image that acts as the preview image for your video. It’s kind of like a book cover. And, like a book cover, it should entice a potential viewer to want to see more. And the same is true of your video thumbnail
It is important to make your thumbnails as engaging as possible while also using your thumbnail to tell a potential viewer what the video will be about. Because YouTube is primarily a visual platform where people watch videos, viewers will look at thumbnails before they read the title of your video