RBAC yaml syntax/structure of Rolebinding group to system:masters

Question

Update: syntax definition - I am aware that it is just a name, but it is some structured name/string.

In the following rolebinding, what is the colon in subject: - apiGroup"system:masters" doing - it is not a url, is it an YAML reference syntax standard way?

kubectl get clusterrolebinding cluster-admin -oyaml
apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRoleBinding
metadata:
  annotations:
    rbac.authorization.kubernetes.io/autoupdate: "true"
  labels:
    kubernetes.io/bootstrapping: rbac-defaults
  name: cluster-admin
roleRef:
  apiGroup: rbac.authorization.k8s.io
  kind: ClusterRole
  name: cluster-admin
subjects:
- apiGroup: rbac.authorization.k8s.io
  kind: Group
  name: system:masters