This read-only Unsigned64 holds the raw count of FCS (CRC) errors -- frames received on this interface whose frame check sequence doesn't match their contents, meaning the frame was corrupted somewhere between sender and this port -- for this interface's live, currently-active error-statistics record. This is the textbook signature of a degrading SFP, a marginal cable run, or electrical interference specifically on this link, as opposed to a congestion-related counter. For example, a steady trickle of a handful of FCS errors an hour might be tolerable, but thousands appearing in one interval on this port would justify swapping its transceiver first.