This counts VRRP packets received with an authentication type field this router doesn't recognize, relevant on deployments that still use VRRP's legacy authentication mechanisms, largely deprecated in favor of relying on the mandatory TTL-255 check for security in modern VRRP. A nonzero count here on a network where all VRRP peers are supposed to be configured identically for authentication is a sign one peer's authentication configuration doesn't match the others, or that unexpected/unauthorized VRRP traffic is reaching this router's interface. Since modern deployments often don't rely on VRRP authentication at all, seeing this counter increment is also worth checking against whether legacy authentication was even intentionally configured on this network.