It is a device which can provide a load in order to allow a power supply to be tested.
Sorry if that sounds obvious, but that's what it is.
Typically it provides a known load, either by providing one or a combination of (possibly variable) current, resistance, or power loads. If used to test a current source, it may provide a resistance, power or voltage load. If used to test a maximum power regulator (as used in solar installations) it may provide a resistance, current, or voltage load.
Other combinations don't make sense (e.g. a constant voltage load for a voltage source, constant current for a current source, or constant power to a maximum power source)
Essentially it acts like a big fat resistor, but the resistance may change in order to maintain the characteristics of the load you wish to emulate.