A baking stone is a flat stone

Rock (geology)

In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...

 that may be used when baking

Baking

Baking is the technique of prolonged cooking of food by dry heat acting by convection, and not by radiation, normally in an oven, but also in hot ashes, or on hot stones. It is primarily used for the preparation of bread, cakes, pastries and pies, tarts, quiches, cookies and crackers. Such items...

. Food is put on the baking stone, which is then placed in the oven, though sometimes the stone is heated first. Baking stones are used much like cookie sheets, but may absorb additional moisture for crispier food. Due to the thermal properties of the stone used, food is less likely to burn when using a baking stone than when using a cookie sheet.