A tripod is only as sturdy as the ground it is on. If you are working on soft ground such as grass, moss, sand etc, then spikes offer far more support for your camera. Once you have set your tripod in the perfect place, press down so that the spikes dig into the ground. Your camera will now be far more stable than if you had used standard rubber feet. These come in a set of three and are available in stainless steel or alloy. The stainless steel ones are more robust and will last longer, but the alloy ones are lighter.
There are two different lengths. The standard ones are 4cm in length (not including thread) whereas the heavy duty ones are 5.5cm.
Price is for a set of three.