One watt-hour corresponds to the work of a machine with an input power of one watt in the course of one hour.
In the context of NETIO products
NETIO smart sockets measure the electricity consumption at each output in Wh. The smart sockets device behaves as an electricity meter, and remembers the previous consumption even when it is powered off. Consumption counters can be reset manually in the web interface.
The consumption counters are 4-byte values; they overflow at 4294967296 Wh. This corresponds to 4.294967296 GWh; assuming 0,20€ per kWh, 858 993 EUR worth of electricity would have flown through the output.