Here is a brief commentary on the book of Joel.

1. The book begins with a section describing a horrible plague of locusts, and more locusts, and more locusts. (Joel 1:1-2:11)

Now I am guessing that this is referring to the time period that Israel is destroyed be the Assyrians and Judah by Babylon. The armies are the locusts.

2. Then Joel asks for Israel to repent and turn back to God. (Joel 2:12-17)

This fits with what the other prophets have been asking of Israel. Hosea, Jeremiah, Ezekiel, etc. So this seems to reinforce the previous understanding of the locusts.

3. And then Joel gives some good news about God restoring Israel and the land ravaged by the locusts. (Joel 2:18-26)

This is also very similar to the other Prophets. That God is only punishing Israel so that they will come back to Him.

4.And then a prophecy that is fulfilled in the New testament. (Joel 2:28-32)

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.
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.

— Joel 2:28-32

The big news here is the idea that ‘everyone who calls on the name of the Lord will be saved’. Israel wasn’t understanding that God loves all people, they really thought he loved them, and them alone. God is challenging that view. And the Holy Spirit had been only for some very unique people. namely prophets and Kings. But Joel says that the Holy Spirit will be given to servants and to men and women alike. Men and women alike would have been a fairly revolutionary concept at the time. Again, God is challenging what Israel believes.

5. And then a prophecy that appears to clearly be about the end times. (Joel 3)

I can’t say I really understand much about this chapter but I think it is about events yet to come. And since I really don’t understand it I won’t say much about it. If your interested in it you can click on this link to Joel Chapter 3.