Wp-includes.bak is essentially a backup directory created by WordPress during updates or when performing certain maintenance tasks. It holds a *copy* of the original `wp-includes` directory, acting as a safety net should anything go wrong during the update process. The `wp-includes` directory is a core component of WordPress. It contains a vast collection of files that are crucial for the proper functioning of your website. These files encompass various essential aspects, including: * **Core WordPress functionality:** Files relating to the underlying structure and operations of WordPress, such as the database interaction, user authentication, and core functions used throughout the CMS. * **Template Tags:** These are pre-built PHP functions that theme developers use to retrieve and display data from the WordPress database within their themes. Without these tags, themes couldn't effectively pull content and information. * **Widgets:** The files necessary for handling the functionality of WordPress widgets, allowing users to easily add pre-built elements (like search bars or recent posts lists) to their site's sidebars, footers, or other widget-ready areas. * **JavaScript libraries:** WordPress uses various JavaScript libraries (like jQuery) to power interactive elements and dynamic features on the website. The `wp-includes` directory holds the necessary JavaScript files. * **CSS stylesheets:** While themes primarily handle the visual design of your site, `wp-includes` contains certain CSS files that provide default styling for some core WordPress components. * **Internationalization (i18n) files:** These files manage the translation of WordPress into different languages, ensuring that the WordPress interface and core functionalities can be displayed in the user's preferred language. During a WordPress update, the existing `wp-includes` directory is replaced with a new version containing updated files. Before this replacement happens, WordPress intelligently creates `wp-includes.bak` to archive the old files. So, what's the purpose of having this backup? If the update process is interrupted or encounters an error (e.g., due to plugin conflicts, server issues, or insufficient permissions), the new `wp-includes` directory might be incomplete or corrupted. In such a scenario, the `wp-includes.bak` directory provides a crucial restore point. You can simply rename `wp-includes.bak` back to `wp-includes`, effectively reverting to the previous version and restoring the functionality of your website. It's generally safe to delete the `wp-includes.bak` directory *after* you've confirmed that the WordPress update was successful and your website is functioning correctly. Leaving it there unnecessarily takes up disk space. However, it's always wise to have a complete website backup before deleting *anything* in your WordPress installation, just in case. Deleting `wp-includes.bak` *before* confirming a successful update is risky because you lose the ability to easily rollback if something goes wrong. The existence of `wp-includes.bak` buys you time and provides a safety net in case of unexpected issues during the update process. Ultimately, `wp-includes.bak` is a testament to the robust and cautious approach taken by WordPress to ensure data integrity and minimize the risk of website downtime during updates.