FIPS stands for Federal Information Processing Standards and you can read more about it at https://en.wikipedia.org/wiki/Federal_Information_Processing_Standards.
Note
This is an enterprise-level feature of Security Onion. Contact Security Onion Solutions, LLC via our website at https://securityonion.com/pro for more information about purchasing a Security Onion Pro license to enable this feature.
The recommended way to use FIPS with Security Onion is to install via our Security Onion ISO image. At the ISO boot menu, you'll need to modify the boot command. This can be done using the e key in UEFI boot mode or the Tab key in BIOS boot mode before it automatically boots. Then append fips=1 (and possibly other options like :ref:`luks` and :ref:`stig`) to the boot parameters and continue the boot process.