The Activity indicator has the value yes(1) when the hardware has recently detected network traffic on the associated network port, and no(2) otherwise.
Because the individual chips and protocol rules that are used to process network signals vary, the precise meaning of the Activity indicator (i.e. what constitutes "traffic") varies from model to model.
On certain variants of certain products, the chipset is unable to detect/report network activity. In these cases, the Activity indicator has a value of notSupported(3).