For example, we have the class Rating that contains score, review, and comment fields. We also have a Movie class. So we can create a record of class Rating that will point to the record of the class Movie via its _parent_id field, so the _parent_id field will contain the ID of record from class Movie.
This is not a simple soft link. This is actually a hard link. When you delete the Movie class record then all its children (records of class Rating with _parent_id field set to the Movie class record ID) will be automatically deleted as well.See this request to learn how to create a child custom record.
If you need to retrieve all children, you can retrieve records with the filter _parent_id=<id_of_parent_class_record>.