What does the prophecy of Joel mean?
You may need to narrow down your question. The prophecy of Joel covers three chapters and 73 verses. Which prophecy of Joel are you referring to? Let me take a stab at a broad answer.
Basically, the prophecy of Joel is about the need for repentance and the coming of the “Day of the Lord.” This is a broad concept, which includes God’s coming in judgment on his people or on the enemies of God. The “Day of the Lord” is usually a day of judgment and of suffering because of a stubborn refusal to repent, but in some cases it can be a day when God comes to pour out blessings on his people. That certainly was the case in Acts chapter 2, which quotes from Joel, identifying the events of Acts 2 with the prophecy in Joel 2:28f. A suggestion: Read Joel 3:13-16 and Joel 3:19-21. In these passages I think you will get a good feeling for the theme and message of Joel. Here we see the time of harvest, but of harvest for judgement because of sin. We also see people “in the valley of decision” about whether or not to repent and turn to God. God will roar and he will thunder on the Day of the Lord, but those who rely on God will find a refuge. Egypt (symbolic of sin and those enslaved to sin) will be desolate because of the evil they did to God’s people, but Jerusalem will be inhabited forever. These words were partially fulfilled at Pentecost, but they will be fulfilled in the greatest sense on the day of final judgment.