WordPress 4.9.1 is a security and maintenance release for all previous versions.
There are four security issues addressed in this update as well as eleven bugs that were fixed. Please make sure you update your sites if you do not have auto updates enabled.
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.