What is the meaning of Thumbnail?

The fingernail on the thumb.

A rough sketch (e.g., the size of one's thumbnail).

A small picture, used as a compact representation of a larger image.

A picture displayed by posts and videos pre-clicked conveying the matters discussed/shown in that post.

To describe concisely.

To create a smaller representation of (a larger image).

thumbnail (a miniature preview of a larger image)

Source: wiktionary.org