Clay tablets were a medium used for writing. They were common in the Fertile Crescent, from about the 5th millennium BC.
A clay tablet is a more or less flat surface made of clay. Using a stylus, symbols were pressed into the soft clay. It is possible to correct errors on the tablet. The tablet was then baked until dry and hard, either by leaving it out in the sun, or in a fire. Sun-baked tablets could be moistened and recycled. The ones that survived thousands of years were baked in fire. They were used by the Sumerian civilisation in Mesopotamia.
Cuneiform was the first writing used on clay tablets. The system was used by several civilizations who spoke several different languages.