The Related Items module displays other Content Items that are related to the Item currently displayed. These are based on the keywords Metadata. All the keywords of the current Content Item are searched against all the Metadata keywords of all other published items. For example, you may have an Item on 'Breeding Parrots' and you may have another on 'Hand Raising Black Cockatoos'. If you include the keyword 'parrot' in both Items, then the Related Items module will list the 'Breeding Parrots' Item when viewing 'Hand Raising Black Cockatoos' and vice-versa.