Nahum was a prophet (holy man). He wrote about a country which was an enemy of Israel, called Nineveh. The people in Nineveh were very evil (Nahum 3:1).
The people from Nineveh were also cruel to the people in other countries. But God cares about the people who trust him (Nahum 1:7). So, God would send the enemies of Nineveh to destroy Nineveh.
God would end Nineveh's cruelty. But God would help his own people, Israel. And Israel would become a great nation again (Nahum 1:15).
The Book of Nahum is in the Bible (Old Testament).