This release contains 23 fixes and improvements as well as contains a number of security fixes too.
You can read more by looking at the release notes: https://wordpress.org/support/wordpress-version/version-5-4-2/
You can also see a complete list of changes: https://core.trac.wordpress.org/query?status=closed&resolution=fixed&milestone=5.4.2&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.