nrpe is used to execute Nagios plugins on remote hosts and report the results
to the main Nagios server. From the Nagios homepage:

Allows you to execute "local" plugins (like check_disk, check_procs, etc.) on
remote hosts. The check_nrpe plugin is called from Nagios and actually makes
the plugin requests to the remote host. Requires that nrpe be running on the
remote host (either as a standalone daemon or as a service under inetd).