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 closed 15-minute history bin -- a fixed, final count for one completed interval rather than something still changing. For example, over that completed 15-minute bin, 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.