I can’t find documentation about this on the site or through the command line with --help. Thanks.

It specifies the directory where the Prince config and data files are found, typically /usr/lib/prince. If not specified, the Prince binary will just look in the parent directory, so if the binary is /opt/prince/bin/prince, it will look in /opt/prince. Normally on Linux systems there is a shell script in /usr/bin/prince that calls the binary with a full path, so the --prefix option is not necessary.