Nrpe, or Nagios Remote Plugin Executor, is the client side service of a monitoring setup. The monitoring server will send commands to the client, which listens passively when got no work to do. Upon incoming command, the nrpe checks it's local configuration, and executes the plugin configured with the command, then sends back the results to the server for processing. You can read more about the server side installation in the Nagios installation guide, and this guide will focus on the client side.
This is a companion discussion topic for the original entry at https://linuxconfig.org/how-to-setup-nrpe-for-client-side-monitoring