In computing terms, Daemons is a process that runs in the background. Hadoop has five such daemons, namely NameNode, Secondary NameNode, DataNode, JobTracker, and TaskTracker. Each daemons runs separately in its own JVM. To know more about NameNode, Secondary NameNode and DataNode, check out this Hadoop Tutorial by Intellipaat.
Also, watch this YouTube tutorial on Hadoop: