The WordPress register_taxonomy function is a built-in WordPress function that allows users to create custom taxonomies for their WordPress site beyond the default categories and tags that are included with WordPress. To use the register_taxonomy function, users need to provide several arguments to define the properties of the custom taxonomy. These arguments include the name of the taxonomy, the types of objects that the taxonomy will be applied to (such as posts or pages), and labels for the taxonomy (such as the singular and plural forms of the term). Once the custom taxonomy has been registered using the register_taxonomy function, it can be used in a similar way to the default categories and tags. Users can create new terms within the taxonomy, and assign those terms to individual pieces of content on their site. This allows for more fine-grained organization and classification of content.