When baking bread rolls or other small baking goods, steam is important so that the dough can rise in the oven. The humidity prevents the oven heat from drying out the top of the bread. That way the dough can keep rising.
The bread rolls will rip open on the sides or bottom when there is too little humidity.
The steam is released often after half of the baking time to allow a crust to form.
Some ovens have a steam function built-in. Professional bakers use these to save time and effort.
When baking bread instead of bread rolls it's best to use a dutch oven. The humidity that is contained in the dough is enough to let the dough rise inside the pot. The humidity is contained in the much smaller pot compared to the oven. For bread rolls however that doesn't work.
In general, any material can be used that can heat up to high temperatures and have a high surface area so that it can create lots of steam when water hits it. Some people use stones, however that can give off dust. Water can also be thrown on the bottom of the oven directly. This can cause calk spots on the oven and could damage the oven because of the big temperature difference.
Steam is very hot and can burn skin easily. With any method of creating steam it's good to make sure that it's save.
One idea to create steam in an oven is to use a lot of metal balls inside a metal cup because it can get hot and has a large surface area. This method is safe because the water is injected through a tube with an almost closed oven door.