Component is a part of the service. For example, HDFS service consists of the following components: Client, DataNode, JournalNode, HttpFS server, NameNode, ZKFC. Breaking a service down into components makes it easier to work with the complicated cluster software.
The set of components depends on the bundle that is used to deploy a cluster. After cluster is created and necessary services are added, you can choose components at the Hosts - Components tab.
Hosts - Components tab in ADCM