Packet loss is the fraction of packets sent across a network that never reach their destination. It is usually expressed as a percentage and measured over a window of seconds to minutes.[1]
The most common causes are congestion at a bottleneck, faulty cabling, overloaded queues on a router, and wireless interference. Loss that appears only under load usually points to a buffer that is too small rather than to a broken link.
Counters are read from interface statistics — rx_dropped, tx_dropped and the per-queue drops. Compare them with the total packet count over the same interval; a single spike right after a reboot is expected.
Increase queue depth, enable a modern congestion control algorithm, and avoid oversubscribing uplinks. Where loss comes from the last mile, nothing on the server side will fix it.