A group is simply a page that matches a particular pattern1, typically just ending in the word "Group" and containing a number of bullet points. The bulletted items are the users in the group. You can use a group in a fashion similar to a user when creating access control lists. So, if you were to restrict a page to be only edited by FashionGroup, then the users who are marked as bullet points on page FashionGroup would be able to edit.
- 1The config value is page_group_regex