What is the "kube-system" namespace for?
kube-system is the namespace for objects created by the Kubernetes system.
Typically, this would contain pods like kube-dns, kube-proxy, kubernetes-dashboard and stuff like fluentd, heapster, ingresses and so on.
kube-system contains service accounts which are used to run the kubernetes controllers. These service accounts are granted significant permissions (create pods anywhere, for instance). Since openshift builds on top of kube, we inherit the structure.
You should avoid putting anything "personal" in that namespace since kube considers it to be "owned" by kube and the permissions for the SAs inside are quite high.