Electronic loads are used to draw power from a source at either a constant current or a constant voltage. This comes in useful for things like battery discharge testing or making sure that PCB you designed can actually power those motors without releasing the all important magic smoke.
The kit is sold on Tindie:
MightyWatt turns your Arduino Uno R3, Arduino Zero (M0/M0 Pro) or Arduino Due into an electronic load capable of dissipating 70 Watts in a very small form factor. Ideal for testing power supplies, batteries, fuel cells or power amplifiers.
Design files and source are shared on GitHub: