Rocks are heavyish collections of minerals. They are naturally-occurring, and can be any size. Most are below-ground.
Whether or not a rock is dangerous depends largely on whether it's above you—or whether it's been thrown at you. If someone throws a rock at you, that is a form of violence and in such an instance the rock presents somewhat of a hazard related to its size and the force behind the throw. It could easily hurt.
Aside from those situations, rocks are pretty benign. They don't really do anything. Some rocks, though, are inherently dangerous because they are radioactive. We advise you to stay away from these rocks.
Lastly, walking on rocks can be a little more dangerous than walking on dirt or grass. If you're walking on rocks... be careful!
Because they do very little, rocks are usually not too dangerous.
