This counts packets dropped specifically because their IP TTL, or time to live, reached zero before reaching their destination. A rising count here usually indicates either a routing loop somewhere in the network causing packets to bounce until TTL exhausts, or simply traffic originating with an unusually low starting TTL that cannot survive the number of hops in this path, worth distinguishing since the fixes are very different. A sudden jump in this counter coinciding with a recent routing change elsewhere in the network would point strongly toward an accidental routing loop rather than a traffic-pattern change.