This release addresses security issues as well as includes changes to help site owners prepare for a minimum of PHP 5.2.
You can read more by looking at the release notes: https://codex.wordpress.org/Version_5.1.1
You can also see a complete list of changes: https://core.trac.wordpress.org/query?status=closed&resolution=fixed&milestone=5.1.1&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.