This read-only gauge is the current number of multicast group entries for this interface in the Cache Table. As a gauge rather than a monotonic counter, it directly reflects the real-time number of active multicast groups being tracked on the interface, useful for capacity planning and spotting abnormal group churn. For example, an admin seeing igmpInterfaceGroups suddenly jump to an unusually high number on an interface might investigate a misbehaving multicast source or a join-flood condition.