You will find in that section mechanical parts to build your robots. If microcontroller is the brain and actuators are muscles, we can say that mechanical parts are bones. They are pretty much inactive in themself, they are used to create the robot's frame and will need actuator to move, run, turns or do whatever you want. Weight and solidity and size are some factors you may consider in your purchase. They can be made of plastic, composite, aluminum for instance.