A rock plate is a rigid layer within the midsole, made of either plastic or a carbon fiber composite material. It’s designed to protect the feet from sharp stones and harsh hazards on rough ground.
It’s found between the midsole and the outsole of the shoe. That way, the wearer still gets the full benefit of the midsole foam and the traction of the outsole, but has a subtle extra layer of protection.
They vary in thickness and rigidity, often depending on the brand of shoe you choose. What you choose depends on you, your comfort, and the surface you run on!