If an electric circuit has a well-defined output terminal, the circuit connected to this terminal (or its input impedance) is the load. (The term 'load' may also refer to the power consumed by a circuit; that topic is not discussed here.)
Load affects the performance of circuits that output voltages or currents, such as sensors, voltage sources, and amplifiers. Mains power outlets provide an easy example: they supply power at constant voltage, with electrical appliances connected to the power circuit collectively making up the load. When a high-power appliance switches on, it dramatically reduces the load impedance.
If the load impedance is not very much higher than the power supply impedance, the voltage will drop. In a domestic environment, switching on a heating appliance may cause incandescent lights to dim noticeably.