The media attachment page is a page including the header and the footer of a wordpress installation and a media as only item.
Here is a URL of a picture that was submitted to a WordPress installation:
This picture is available on this page:
WordPress automatically creates a media attachment page for this picture. When the Yoast plugin is not installed, the following page can be accessed. This page URL is made of the page where the media can be found plus the title of the image URL.