Say you have a category field where every record in your table stores either "animal", "Vegetable" or "mineral" and you want to see a list of all records with the same category as the current record in a portal.
In Manage | Database | Relationships, drag from YourTable::Category to outside the box and then back to the Category field before releasing the mouse button. Name this new "instance" of a table occurrence "SameCategory".
You now have two occurences of the same data source table related by Category:
YourTable::Category = SameCategory::Category.
Now place a portal to SameCategory on your YourTable layout and you've created and implemented your first self join relationship.