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 today's still-accumulating 24-hour 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 24-hour 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.