Thumbnails are small images that are used to represent a larger image or video. They are often used on websites and social media platforms to give viewers an idea of what the content is about before they click on it. The term 'thumbnails' can also be used to describe small pictures that are printed on the back of a book or magazine. The crossword solution for the term 'thumbnails' is 'tacks, ' which refers to the small pins or nails used to attach the thumbnail to the larger image or video.