What is QuickLookThumbnailing on macOS?

/System/Library/Frameworks/QuickLookThumbnailing.framework/Versions/A/QuickLookThumbnailing

Generate thumbnails for common file types and add a Thumbnail Extension to your app to enable others to create thumbnails of your custom files

You may want to create a miniature representation, or thumbnail, of a file and its contents to display within your app. The QuickLookThumbnailing framework provides an API to generate thumbnails using the QLThumbnailGenerator object. It can generate thumbnails for common file types.

source: developer.apple.com