This release offers a number of refinements and introduces the Twenty Twenty theme.
Block changes, a new theme, improved image upload, automatic image rotation, improved site health checks, as well as interface changes are just a few of over 150 new features that WordPress 5.3 has to offer. If you have been holding back from working with the editor – it is time to give it another try.
You can read more by looking at the release notes: https://make.wordpress.org/core/5-3/
You can also see a complete list of changes: https://core.trac.wordpress.org/query?status=closed&resolution=fixed&milestone=5.3&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.