Heatsinks make it easier to move waste heat away from where it could be dangerous. The microchip that drives a stepper motor in a robot has so much current flowing through it that it can get burning hot. To keep temperatures down, a small metal heatsink will draw the heat away from the chip. The fins increase the surface area through which the heat can be absorbed by the air. Combine with a fan for best results.
This is a pack of 5. Comes with adhesive for easy installation.