The prophets speak on behalf of the Lord to His people. They confront the people about how they have broken the covenant, warn of the curses that God will send on them (and on all nations), and promise blessings for Israel in the latter days. These promises find their ultimate fulfillment in Jesus’ kingdom. Right now it takes faith to see that they are fulfilled, and when Jesus returns they will be more visibly fulfilled.
In Joel’s short book, it is easy to see the contrast between current curses and future blessings. Currently, the people are not able to offer grain and wine to the Lord, because the grain, wine, and oil are destroyed (1:9-10). This should lead the people to fast and pray (1:14). The day of the Lord, further judgment, is coming (2:1-11), but if the people repent they may be blessed with a grain offering and a drink offering (2:14).
Joel 2:18-32 describes blessings that the Lord will bring in the latter days, motivated by His jealousy and pity for His people. He will send rain to produce grain, wine, and oil. The point is that He is undoing the curse and bringing blessings instead (see 2:25). This salvation is final; Israel will never again be a mockery among the nations (1:19) or be put to shame (1:27). This finality is part of why I understand this passage to be fulfilled in Jesus’ kingdom and not sooner. The meaning of the grain, oil, and wine is that there will be blessing instead of curse, and the people will be able to worship the Lord. Jesus’ death and resurrection, motivated by God’s jealousy and pity for his people, removes his people’s curse so they can share in the blessings he deserves. His jealousy led him to buy us as His permanent possession, and His pity led Him to give us everlasting salvation.
Then “afterward” (2:28), when the curse is undone and God’s blessings are flowing, the Spirit is poured out. He is the greatest blessing we receive in this life, and the guarantee of every everlasting blessing.