A Kubernetes cluster consists of a set of servers. These servers can be VMs or physical servers. The latter are also called bare metal. Each member of the cluster can have one of two roles. It is either a Kubernetes master or a (worker) node. The former is used to manage the cluster, while the latter will run an application workload. I have put the worker in parentheses since, in Kubernetes parlance, you only talk about a node when you're talking about a server that runs application workloads. But in Docker parlance and in Swarm, the equivalent is a worker node. I think that the notion of a worker node better describes the role of the server than a simple node.
In a cluster, you have a small and odd number of masters and as many worker nodes as needed. Small clusters might only have a few worker nodes, while more realistic clusters...