When Solomon's son Rheaboam became king, he treated the people very harshly. God was not happy with the way this king was treating his people, but he had to keep Rheaboam as king because of a promise given to his grandfather David.
So, God split 10 tribes away from this king, and they became the ten-tribe kingdom of Samaria (Israel) ruled by king Jereboam.
The other two tribes, known as the two-tribe kingdom of Judah, stayed under the rule of Rheaboam.