Joel was a prophet (holy man). The Book of Joel is in the Old Testament part of the Bible.
The Book of Joel describes some terrible events. Joel called these events 'the Day of God'. A terrible army would attack Jerusalem. They would destroy everything. Nobody could escape from this army.
So, the people must confess their evil deeds to God. Even when these terrible things happen, they must ask God to save them (Joel 2:12-17).
Then, God will be kind to his people. He will hear their prayers. He will cause this terrible army to leave. He will provide food for his people. And he will send his Spirit on all people (Joel 2:28-32).
God will also punish the nations that attack Jerusalem (Joel 3:1-16). But God will protect his own people always (Joel 3:17-21).