Humans are not the only ones who rely on a cluster. Processes in containers often need to invoke Kube API as well. When using RBAC for authentication, we need to decide which users will have permissions to perform specific actions. The same holds true for the processes running inside containers.
When we (humans) try to access a Kubernetes cluster with RBAC enabled, we are authenticated as users. Our username provides an identity that API server uses to decide whether we are allowed to perform intended actions. Similarly processes running inside containers might also need to access the API. In such cases, they are authenticated as a specific ServiceAccount.
ServiceAccounts provide a mechanism to grant permissions to processes running inside containers. In many ways, ServiceAccounts are very similar to RBAC users...
