Joel, like many prophets of the Old Testament, called the people of Israel to repentance. He warned his people of the possibility of judgment for their unfaithfulness, and some see in his words a prediction of the ultimate day of judgment.
He draws on recent events to provide an image to underscore his message. The analogy is to locusts, and outbreak of which had stripped the land of anything green and caused a famine. God’s judgment would be similar: sudden, swift, devastating and unstoppable.
A judgment of this type will eventually be applied to all people. Unfortunately, a lot of people will be taken by surprise. In the time of Joel, God preserved a remnant of the people to survive, and they turned their hearts to Him. In a similar manner, God has called out to people throughout history, and in our age the Gospel is a message of His salvation to all who will accept Christ.