In a normalized database, we store separate entities in separate tables and define relationships between them, those being - one-to-one, one-to-many and many-to-many. We often want to fetch data which is split over two or more related tables, which is dead simple when fetching the extra data from the 'one' side of a relationship (ie, from a "parent" table):
SELECT product_images.src,products.name FROM product_images INNER JOIN products ON product_images.parent_id=products.id
Literally: Get the name of the parent product for each row in the products_images table.
Going the other direction is not as trivial. If we just want all the referenced rows in the child table, the query seen before would do the trick. But if we wanted a specific row to be found - for example, the latest inserted image, it becomes somewhat more complicated.