What is this code add_to_serializer in all these plugins

Heyy, I have added a custom field to my category edit box.

When I submit the form, I can see something these fields are submitted in the js console
custom_fields[default_tag][]:good-feedback custom_fields[default_tag][]:one-more-tag

in my main plugin file, I am doing

add_to_serializer(:basic_category, :default_tag){object.custom_fields[“default_tag”]}

My custom fields aren’t getting saved to the db.