So the alias of the article is what is included in the url to make each page unique. It is also called a "slug." The alias is usually auto generated from the title based on some preset rules and most people don't need to set the alias.
The alias prevents two articles by the same name from getting mixed up on your profile. So the article "untitled" by zany lynn is fine. But if you submit another different article with the same name, the website will try to construct that link with the same alias and find a conflict. Each alias must be unique. So the alias for one may by "untitled" and the other may be "untitled-2" for instance. On your profile you will have two articles titled "untitled" but they will have two distinct urls since they are two distinct articles.
This problem seems to crop up most when users resubmit an article many times before the initial one is moderated. Once the new article is moderated and approved, the article can be edited many times by the user without having to change the alias. But if, while the article is in moderation status, the same article is resubmitted 10 times, the system will need to make (or the user will have to specify) 10 different aliases.
Finally, moderators oftentimes set aliases behind the scenes to make sure everything is unique. I hope this helps.
Joshua Hennen
