This field reports the cumulative count of physical-layer symbol errors — invalid line-code symbols the PHY detected on the wire, tracked by this particular stats object since it was created or last reset. For a network admin, on this interface a rising symbol-error count almost always traces back to a marginal cable, connector, or SFP rather than anything happening at the Ethernet MAC or above. For example, a cumulative value of 15 symbol errors since the interface came up would still be within a tolerable range, whereas a rapidly climbing count in the hundreds would justify escalation.