This counts IP multicast datagrams destined for this specific group address that have been forwarded out this specific interface, per its description, the outbound counterpart to igmpCondFilterMcastInPkts. Comparing In and Out for the same group-and-interface row tells an admin whether Conditional Filtering is actually passing the traffic through or silently dropping it despite packets arriving. If InPkts is climbing steadily but OutPkts stays at 0, that specific interface isn't forwarding the group's traffic, likely because igmpCondFilterMcastMember or the router-presence check is false for that row.