I think that this book is is telling people get up and tell the world. To tell the world about God and his coming. I think it is telling us to also prepare for his coming so that when he comes we are ready and we can with him to live with him forever.
The word of the Lord that came to Joel the son of Pethuel. Hear this, ye old men, and give ear, all ye inhabitants of the land. Hath this been in your days, or even in the days of your fathers? Tell ye your children of it, and let your children tell their children, and their children another generation.
Rejoice, you people of Jerusalem! Rejoice in the Lord your God! For the rain he sends demonstrates his faithfulness. Once more the autumn rains will come, as well as the rains of spring.
But everyone who calls on the name of the Lord will be saved, for some on Mount Zion in Jerusalem will escape, just as the Lord has said. These will be among the survivors whom the Lord has called. The Lord’s voice will roar from Zion and thunder from Jerusalem, and the heavens and the earth will shake. But the Lord will be a refuge for his people, a strong fortress for the people of Israel.
“Then, after doing all those things, I will pour out my Spirit upon all people. Your sons and daughters will prophesy. Your old men will dream dreams, and your young men will see visions. In those days I will pour out my Spirit even on servants—men and women alike. And I will cause wonders in the heavens and on the earth— blood and fire and columns of smoke. The sun will become dark, and the moon will turn blood red before that great and terrible* day of the Lord arrives.