It limits journalling of messages forwarded to administrators via the special SMTP 'postmaster' or 'abuse' addresses.
Setting JournalLimitForwardToAdmins=1 will prevent this duplicate copy of the forwarded message from being journaled.
The original message will still be journaled. This NOTES.INI setting must be enabled on the same server which has journaling enabled, and which is doing the forwarding.
Recommendation: Enable this setting on servers with journalling enabled to substantially reduce the size of the journalling database.
Domino Configuration Tuner: This setting is included in the DCT catalog of best practices as of Domino 8.5