Kerberos is an authentication protocol developed at MIT. It uses tickets to ensure that a service/node/user are who they say they are and allow access to each other.
You can read all about Kerberos on MIT's website: https://web.mit.edu/kerberos/
Microsoft Active Directory uses Kerberos under the covers to provide ticket based authentication for services. You can integrate your cluster with AD using Kerberos to provide access to various services running on the cluster.