Major features for WordPress 5.1
Before the scheduled release of WordPress 5.1 on February 21st, the WordPress core team has released the WordPress 5.1 release candidate. This release candidate signifies that the WordPress 5.1 is ready to be shipped, after incorporating bug fixes in the beta versions 1, 2 and 3. It is made available for the public to test to ensure the stability of the WordPress 5.1 update.
There are two ways for you to check out the WordPress 5.1 release candidate: (1) install the Beta Tester plugin on your site and manually update to the latest build; (2) or, download its zip file. The core team encouraged developers to test the latest build as well as check their plugins and themes compatibility with WordPress 5.1. Developers can report bugs and other issues by creating a ticket or posting in the forum.
One of the major updates of 5.1 is the Site Health which will prompt site administrators to upgrade to the latest PHP version. As WordPress is powered by PHP, updating the site, themes and plugins to the latest PHP version will prevent fatal errors in the future.
The Site Health feature will also post a notification when a plugin is incompatible with your site or uses outdated versions of PHP, and block its usage.
Improved Gutenberg Editor
For the full list of features, check out the WordPress 5.1 Field Guide.