A cluster is a collection of distributed computer nodes that are connected by a network in some way, often using Local Area Network (LAN). The purpose of the connected nodes is to efficiently handle tasks by communicating with each other and sharing or dividing work and information among themselves. From an onlooker's perspective, the cluster acts like a single computer, as depicted in the following diagram:

Clients calling a cluster of nodes

Its main purpose is, however, to be more (cost-) efficient in terms of, for example, handling more requests and performing more computations compared to a single machine. Extending this to Java EE means that one (or more) application server, such as WildFly, runs on each computer node and in each of these application servers, there are applications or other components (such as EJBs) that execute and work together in order to handle traffic and perform computations.