Explain why president Andrew Johnson was impeached?

He was impeached because adopting 11 articles of impeachment detailing his "high crimes and misdrmanors", in accordance with article two of the United States Constitution

Impeached means accused of a crime. Andrew Johnson was the first president to be impeached. He was accused of breaking the law, because many congressmen thought he was being too nice to the South after the War. He was found not guilty by just one vote. Years later, he became the only ex-president elected to the Senate.