Example:
You’re creating a post with the title “My New Post Title”, and while creating this post you have uploaded the file “Image-1.jpg” from this post edit screen.
When you do this, WordPress creates an attachment page for the image file. Using the above data you may end with the following links:
Your post link: https://example.com/my-new-post-title/
The attachment page for the image: https://example.com/my-new-post-title/image-1/
So basically, WordPress core creates a page to show the image, taking your post slug as base.
If you upload an image directly from the Media Library screen, not within a post/page edit screen, WordPress creates also an attachment page, but this time using the image filename only. e.g. https://example.com/image-1/
What the plugin does is to redirect WordPress attachment pages as explained on the plugin description:
There are three possible scenarios where this plugin does something:
A) A visitor lands on an attachment page and the parent post is available (not in trash or deleted). A 301 redirect will be issued to redirect the visitor to the parent post.
B) A visitor lands on an attachment page and the parent post was already deleted from trash. A 302 redirect is issued to home page (this can be changed to 301 if you want, see FAQ for more details).
C) A visitor lands on an attachment page and the parent post is in trash, therefore not available for the visitor. It returns a 404 error code to prevent endless redirection loop in old WP releases and redirecting to trashed/not available posts.
So when the Google bot is back to your site and see the redirects, it should update the results for your site in Google’s index database, but it’s up to Google how or when to do that. The plugin doesn’t has any control over that process, it simply takes care of doing the redirections.
Attachment pages are created for each unique file only, if you have URLs for galleries, those are not attachment pages, therefore the plugin will not do anything with them.
