This counts frames on this interface where the transmit buffer ran dry before the frame had been fully clocked out onto the wire. A transmit underrun is a symptom of the switch fabric or backplane failing to keep this port's MAC fed fast enough, not a wire-side problem, so a rising TxUnderRunPkts for the current, still-accumulating short interval bin on this specific port is worth escalating to hardware diagnostics rather than to the cable plant. For example, TxUnderRunPkts appearing only on this one port during this in-progress short-interval bin while every other port on the same line card stays clean would point at a fault local to that port's MAC or its share of the backplane bandwidth.