Does exactly what it says on the tin. Basically, it deletes and recreates all the thumbnails in your media library. This is useful because wordpress doesn’t automatically regen your thumbnails when you edit them.
This plugin is hidden under sidebar > tools > regenerate thumbnails. Once you start the regeneration process, it will show a progress bar and a summary of the images it has regenerated thumbnails for.
You can also regenerate individual image thumbnails by going to sidebar > media > image – there should be an option to manually regen thumbnail.
However, be careful when you regenerate your thumbnails – manually set thumbnails will be removed.