Munge daemon needs a secret key file /etc/munge/munge.key.

If not existing previously, the package generates a new key at
installation time otherwise it will use the existing key.

You need to securely propagate the secret key file (eg, via ssh) to
other hosts within the same security realm. Credentials are valid only
within a security realm (ie, on hosts sharing the same secret key). All
munged daemons exchanging credentials need to share the same secret key.