The update addresses a serious security vulnerability that, in certain circumstances, could enable an attacker to gain full access to your Mac and everything on it without a password.
Apple has advised that users of macOS Sierra 10.12.6 and earlier versions are not impacted by the vulnerability that the security update addresses.
Further information is provided by Apple at //support.apple.com/en-us/HT208315
, including links to advice about ‘How to update the software on your Mac’ and find your macOS version and build number.
Stay Smart Online recommends that you always turn on automatic security updates on your computing devices.