In WordPress, a query string is part of a URL that contains data to be passed to WordPress to retrieve specific content from the database. A typical WordPress query string includes parameters such as "p" for post ID, "cat" for category ID, "s" for a search query, and "page" for pagination. The values of these parameters are used by WordPress to determine which content to retrieve and display on the page. Developers can also customize query strings using the WP_Query class to retrieve specific data from the WordPress database. This can be useful when creating custom templates, search forms, or plugin functionality.