Found from Google:
If you see the 'dropped' (second) field being non-zero, your system just can't handle all packets coming in up from the device driver.
PCI bus master unable to get time for writing data out of
internal FIFOs before incoming data overflows those FIFOs.
(Underrun is unability to feed data into transmit-FIFOs
fast enough for packet transmission.)
All in all, if your system has many busmasters (TV frame-grabbber,
busy IDE/SCSI controllers, many network cards), it might do well
to force lower latency values for them all (at BIOS setup.)
Some chips are able to buffer things better than others, and
tolerate longer latencies, or even are able to halt the incoming
flow.