Aliases are virtual 'symlinks' of pages, allowing one page to take on one or more other routes. This is not the same as copying the page, it is a primitive filesystem link which is resolved by Mosaic.
Aliases are great for linking to an old route if you've moved a page to a new place. They can also be used to create short hand links to pages or to 'clone' a copy of a page into another section of the site where it may be relevant.
This is the frontmatter for this page:
Try accessing this page from /mosaic/example/aliases