What baking stone means
A baking stone is a thick, flat slab used in a home oven to copy the hot stone floor of a bakery or pizza oven. It is usually made from ceramic, clay or cordierite, a mineral that copes well with sudden temperature changes, and it may be round or rectangular. A related tool, the baking steel, is a heavy slab of steel used in the same way. The stone is put in the cold oven and heated thoroughly with it, as it takes a long time to get hot.
Dough is then slid directly onto the hot surface, usually from a pizza peel dusted with flour, semolina or polenta, or on a sheet of baking paper. Because the stone stores and releases a lot of heat, the base of the bread or pizza cooks quickly and evenly, giving a crisp, well-browned crust and a lively rise, known as oven spring. Baking stones are popular with home bakers for pizza, sourdough and other crusty loaves, flatbreads, calzone and some pastries. They should not be put into a hot oven cold, or washed while hot, as sudden changes in temperature can crack them.
They darken with use, which is normal. A pizza stone is the same thing, sold mainly for pizza. You will see a baking stone called for in bread and pizza recipes.