Apple released its round of bug fix/security updates - including iOS 11.2.5, macOS 10.13.3 High Sierra, watchOS 4.2.2, and tvOS 11.2.5 - today. In doing so, it also offered some security updates for Macs running older versions of its OS, including OS X 10.11 El Capitan and macOS 10.12 Sierra.
The security updates mainly focus on the Meltdown and Spectre vulnerabilities, which were fixed for High Sierra users a couple of weeks ago. OS X 10.11.6 El Capitan got the smallest update, including fixes for IOHIDFamily, Kernel, QuartzCore, and Wi-Fi.
As for the Sierra update, it's available for machines that are running macOS 10.12.6. It includes the above fixes, but it also includes improvements for Audio, LinkPresentation, Security, and there's an additional Kernel fix.
Obviously, it's recommended that you install these updates as soon as possible. Meltdown and Spectre are significant vulnerabilities, so you're going to want to stay on top of any fixes that are made available now, as well as moving forward.