If you've replaced files with FTP, their properties (file size, dimensions, metadata) may be different. This tool allows you to replace the information stored in the database with that from the new files.
In the case of metadata, certain information (title, description and tags in particular) will only be updated if you have activated the metadata retrieval option (in Features / Metadata).
Resized images are all images created from the original image, such as thumbnails. This tool will delete all these images, allowing them to be recreated automatically. This can be particularly useful if you have replaced files via FTP.