This counts events where the receive-side packet-delay-variation (PDV) jitter buffer ran completely empty because packets weren't arriving fast enough to keep the TDM output continuously fed. Each underrun on this pseudowire forces the device to insert filler/idle data into the reconstructed T1/E1 bitstream -- audible as a click or brief dropout on voice traffic -- so repeated underruns mean the configured jitter buffer is too small for the network's actual delay variation -- and this specific object covers a cumulative counter reset only by an explicit clear rather than on a fixed schedule -- reflecting everything since PM stats were last cleared. For example, since PM counters were last cleared, this pseudowire might show this counter at 25 -- a concrete count the admin can trend against neighboring bins to judge whether the condition it reflects is worsening.