Auto-update plugins and themes, developer changes, sitemaps, lazy load, and block editor changes.
This is the second major release of WordPress in 2020 and it is packed of nice touches that help improve the WordPress experience for users and developers alike. With a big focus on site speed – I would highly recommend checking out the details about the lazy loading implementation. I would also recommend reading about the XML Sitemap implantation. If you have third party plugins that do either of those – it might be worthwhile looking to see if they are still needed.
You can read more by looking at the release notes: https://make.wordpress.org/core/5-5/
You can also see a complete list of changes: https://core.trac.wordpress.org/query?status=closed&resolution=fixed&milestone=5.5&order=priority
If you have not done so already – download the update or simply use your existing installation of WordPress to update your site. Either way – this is an excellent time to make a backup copy of your site(s) before upgrading.