Thumbnails are reduced-size representations of pictures or videos. They function to recognize, organize and manage a group of larger images.
In short, thumbnails give you an idea of what an image looks like.
If your photos, as well as thumbnails, are corrupted and you wish to recover them, you can do that via Stellar Repair for Photo software. The software functions by scanning through the hard drive to look for corrupt Photos. It then repairs the corrupt data in the file, extracts the thumbnail while keeping image and its corresponding thumbnail intact.
To explore the step-by-step process, explore the link here
