With security and speed in mind, this release introduces WordPress’s first Site Health features. WordPress will start showing notices to administrators of sites that run long-outdated versions of PHP, which is the programming language that powers WordPress.
Furthermore, when installing new plugins, WordPress’s Site Health features will check whether a plugin requires a version of PHP incompatible with your site. If so, WordPress will prevent you from installing that plugin.
The new block editor has kept improving since its introduction in WordPress 5.0. Most significantly, WordPress 5.1 includes solid performance improvements within the editor. The editor should feel a little quicker to start, and typing should feel smoother.
The WordPress 5.1 Field Guide has also been published, which goes into the details of the major changes.
This update is expected on 2/21/2019.