A heartbeat in master/slave architecture is a signal indicating that it is alive. A datanode sends heartbeats to Namenode and node managers send their heartbeats to Resource Managers to tell the master node that these are still alive.
If the Namenode or Resource manager does not receive heartbeat from any slave node then they will decide that there is some problem in data node or node manager and is unable to perform the assigned task, then master (namenode or resource manager) will reassign the same task to other live nodes.
