This reports the number of packets currently sitting in this port's outbound queue, waiting to be transmitted, at the instant this object is polled. A consistently nonzero or growing queue depth on a specific port is the leading indicator of that port becoming a bottleneck, showing up before the utilization or error counters necessarily reflect the problem. For example, a queue depth that climbs from 0 to 200 and stays there points to sustained congestion on that specific port that its current bandwidth can't drain.