This is Lachish Letter II, a pottery ostracon with Hebrew inscription. This ostracon is probably a fragment of a wheel-made storage jar.
The so-called "Lachish Letters" are documents consisting of potsherds inscribed in black ink (known as ostraca) which date to the very end of the southern kingdom, when the Babylonians under Nebuchadnezzar II had invaded Judah and were laying siege to Jerusalem. According to Jeremiah, apart from Jerusalem, Azekah, about 18 miles southwest of Jerusalem, and Lachish, 18 miles further on, remained in Judaean hands as the invasion progressed.
The letters were sent from outposts of Lachish to the city's military commander, a man named Ya'osh, and represent field reports monitoring the situation. A total of 21 letters were found, some of them were extremely fragmentary. They were recovered from the ruins of the main city's gate, which was destroyed in 685 BCE.
Iron Age, 6th century BCE. From Lachish, modern-day Israel. (The British Museum, London)