If you are using a released version of Kubernetes, you should refer to the docs that go with that version.
The latest release of this document can be found [here](http://releases.k8s.io/release-1.1/docs/user-guide/security-context.md).Documentation for other releases can be found at releases.k8s.io.
A security context defines the operating system security settings (uid, gid, capabilities, SELinux role, etc..) applied to a container. See security context design for more details.