Validate configuration
To validate current S3fs configuration include configuration inside settings.php file.
File Metadata Cache
This button queries S3 for the metadata of all the files in your site's bucket (unless you use the Root Folder option), and saves it to the database. This may take a while for buckets with many thousands of files.
It should only be necessary to use this button if you've just installed S3 File System and you need to cache all the pre-existing files in your bucket, or if you need to restore your metadata cache from scratch for some other reason.
The file metadata cache keeps track of every file that S3 File System writes to (and deletes from) the S3 bucket, so that queries for data about those files (checks for existence, filetype, etc.) don't have to hit S3. This speeds up many operations, most noticeably anything related to images and their derivatives.
Copy Local Files to S3
When to upload files

Important: This feature is for sites that have configured or going to have configured to take over for the public and/or private file systems. Example: You should have $settings['s3fs.use_s3_for_public'] = TRUE; or $settings['s3fs.use_s3_for_private'] = TRUE; after or before use this actions. You may wish to copy any files which were previously uploaded to your site into your S3 bucket.
If you have a lot of files, or very large files, you'll want to use drush s3fs-copy-local instead of this form, as the limitations imposed by browsers may break very long copy operations.