This statistic measures network interface activity in bytes/sec. Network interfaces are the logical network interfaces, and are shown in the Interface column of Network. The measured bytes by this stastistic includes all network payload headers (Ethernet, IP, TCP, NFS/SMB/etc.)
See Network: Device bytes for an example of a similar statistic with similar breakdowns.
Network bytes can be used a rough measure of appliance load. This statistic can be used to see the rate of network bytes through different interfaces. To examine network devices that make up an interface, especially to identify if there are balancing problems with LACP aggregations, use the Network Device bytes statistic.
Also see Network: Device bytes for network throughput at the device level, instead of the interface level.