References fields are used to model relationship to other content items. For example you could have a schema for products and a schema for product categories. A product has a field with references to the categories it belongs to. Both, products and categories can be created, updated and managed independently. Please think about the direction of the reference very carefully. For example a typical product is only in very few categories, but a product category could have thousand of products. Therefore it is not recommended to reference the products from the categories. When you delete content a cleanup process will remove the referenced id from all contents. This process is executed in the background to improve the performance and it can take several minutes to complete. Therefore it is highly recommended to handle cases where an content has an reference to an deleted content.