The output is given in kW (kilowatt) and corresponds to the heat given off by the stove at a certain time. The more you burn, the greater the heat output. An electric radiator normally gives off an output of 1 kW. An sauna unit normally gives off a full output of 5 kW.
The relationship between the exploited and supplied energy in a system. The higher the level of efficiency (0-100%) the better the stove exploits the supplied energy, in the form of the logs that are burned. A stove that is wood fired usually has an efficiency of 50-70 %. Most Contura models have levels of efficiency in excess of 70%.
If the stove has an output of 4 kW for 10 hours the house has been supplied with an energy volume of 40 kWh (kilowatt hours).