This counter is the running total of IGMP leave messages received, each representing a host explicitly telling the switch it no longer wants a particular multicast group's traffic. Comparing this value against IgmpnumJoins on the same element gives a rough sense of subscription churn -- a large gap where joins vastly outnumber leaves can mean hosts are relying on membership-query timeouts rather than explicit leaves, which delays freeing multicast bandwidth on this port. For example, if joins are in the thousands but leaves stay in the low hundreds, the switch is likely still forwarding several stale multicast streams that nobody explicitly left.