Read this article online.

An electronic load, or simply e-load, is a programmable test instrument that draws controlled amounts of current or power to evaluate the performance and power consumption of power sources.  Electronic loads are widely used to test power supplies, batteries, chargers, DC-DC converters, solar panels, fuel cells, server power systems, and other devices under test (DUTs) that generate or deliver electrical power.

The e-load precisely controls current, voltage, resistance, or power to emulate real-world operating conditions of the DUT while safely consuming the energy supplied by the DUT. The most common type of e-load is called a dissipative e-load.