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 the current, still-open 15-minute interval -- a value that keeps climbing until the interval closes and rolls into history. For example, so far in the current, still-running 15-minute interval, this pseudowire might show this counter at 2 -- a concrete count the admin can trend against neighboring bins to judge whether the condition it reflects is worsening.