This means that your staff will be able to upload (for example) a PDF document that will be attached to a particular web page in your library's site.  For instance, you could attach a PDF version of an event flyer to the calendar listing of the event.  Keep in mind that this is an attachment; it works just like with email.  So the content of your attachment will not show up inline with the other web site text; rather, it will show up as a link that the visitor will click at the bottom of the page content.