Some people are confused about the general meaning of Joel and what it’s significance is for Christians today. I think the biggest key to interpreting the book is the apostle Peter’s sermon connecting the prophecies of Joel to the coming of the Spirit at Pentecost. The book prophetically interprets an invading army as a symbol for the destruction of the temple in 70 AD (cf. Matthew 24) and the outpouring of the Spirit at Pentecost.
The first chapter deals with a locust army that destroyed the crops and the land. The locusts are described four ways: cutting, swarming, hopping, and destroying. Four, symbolically, represents the whole earth, so a world-wide destruction is imagined in the book. Israel is a microcosm of humanity and the world, so the destruction of Israel is tantamount to the destruction of the world. Two of the adjectives immediately strike me as important: priests cut animals to be sacrificed and the swarming sea and air creatures of Genesis 1 symbolize the Gentiles. Locusts specifically reference Gentiles in other places: Judges 6:5, 7:12; Jeremiah 51:14. In a way, these locusts are a priestly nation, cutting Israel into pieces to be sacrificed to God. Not only are these locusts priests, but they are also kings. They have teeth like a lion, the kingly animal of David. Locusts were a punishment on Egypt, and because Israel has become a new Egypt (Deuteronomy 28:38), locusts are an appropriate punishment upon the land. The Apostle John compares the demonic army rising from Abyss as locust-like (Revelation 9), suggesting that these Gentile armies are powered by the demonic anti-Christ forces of Hell.
The loud sound of the chariots and armies symbolized by locusts are a false theophanic Glory-Cloud, which I think are attempting to fill the void left by Yahweh’s exit from the Temple (Ezekiel 10-11). These locust armies bring fire with them, a characteristic of the Spirit (Joel 1:19-20; 2:3-5). Fire can either exalt (Isaiah 6) or burn (Genesis 19), and for sinful Egyptian Israel, it a judgment. Visits from Yahweh can also either exalt or destroy: this army, being an anti-Spirit, comes to destroy the city by scaling the walls and bringing fire against the city. I think Jesus alludes to this passage when notes that the Son of Man will come like a thief in the night because these armies, when they bring fiery judgment on the city, also come like thieves in the night (2:9).
The next section tells Israel how Yahweh plans on restoring her: he will bring bread and wine back to the storehouses and restore oil for anointing. The bread and wine replace the fallen graineries and vineyards, but point toward the Body and Blood of the Lord Jesus. The oil for anointing refers to the ability to consecrate new priests to perform rituals in the land. In the next chapter, God riles up the spirit of the surrounding nations to come to war against him. He will be ultimately victorious over these foreign nations, and restore Israel in a valley that was noted for the deaths that occurred in it. Oil is a symbol of the Spirit in the Old Testament, so God will restore Israel by the Body, Blood, and Spirit.