Related items are the items that have been added to the collection you're viewing. They can include Conversations, people's walls, group walls, Social Objects, or other collections.
Referring items are the Social Object and other collections that the collection you're viewing has been added to (as a related item). Think of them as parent items, where the parent item refers to one or more child items.
To add related item, on the Related tab, click + Add, and select whether to create a new item or add an existing item.
To add referring item, on the Referring tab, click + Add, and select an existing item.
To remove a related item, on the Related tab, click Edit, then click Remove next to the item you want to remove.
For more information, see How do I add or remove a related or referring item?