What the heck is "*Mail Alias:"?

What the heck is "*Mail Alias:"?

Hi,

What the heck is "*Mail Alias:" when I'm trying to setup stuff in People?

Manage Departments
Manage Designations

both areas demand this *Mail Alias: field and I don't know really what it wants. Some sort of email address. I thought it would need this so people could message everyone in that group but if that is the case it should only be optional I would think.

If I have to fill this in what do I put in? The "owner" of the group or just the bosses email?