The member list of an area displays a list of users that belong to the area and normally because they work in that office or department for example for those areas.
Members can, by default, view and publish content to the area but a content reviewer would need to authorise the content before it would display. Intranet administrators often remove members from security roles so it should never be assumed that by being a member of that area, gives you permission to see content in it.
If a user books time off, they can see who else is off in the areas they are a member of.
Users can be added and removed as members from any area
It should be noted that folders do not have a concept of membership.