munin-node is the agent process running on each node that the Munin server monitors. The process itself runs as root, but unless otherwise instructed (through Munin environment files?, usually found in /etc/munin/plugin-conf.d/) it runs the different plugins with a user account with a lower level of privileges.
The agent's configuration file is munin-node.conf, often found in /etc/munin/ or /etc/opt/munin/.
munin-node listens on port 4949. It will respond to regular telnet communication as long as the querying host is listed in munin-node.conf. If you need to (or want to) query the node manually through telnet, see Debugging Munin and the protocol specification for details on how to ask and what to ask for.