As Catholics we are encouraged to read the Bible daily, both publically in Mass and privately. We don’t read the Bible like we do every other book and I don’t just mean reading it as a Holy Book. God had put inside of the Bible different layers which make the Bible a much later book than it already is.
This is called the senses of Scripture. The Catechism of the Catholic Church talks about this in paragraphs 115-118
115 According to an ancient tradition, one can distinguish between two senses of Scripture: the literal and the spiritual, the latter being subdivided into the allegorical, moral and anagogical senses. The profound concordance of the four senses guarantees all its richness to the living reading of Scripture in the Church.
116 The literal sense is the meaning conveyed by the words of Scripture and discovered by exegesis, following the rules of sound interpretation: "All other senses of Sacred Scripture are based on the literal."
117 The spiritual sense. Thanks to the unity of God's plan, not only the text of Scripture but also the realities and events about which it speaks can be signs.
1. The allegorical sense. We can acquire a more profound understanding of events by recognizing their significance in Christ; thus the crossing of the Red Sea is a sign or type of Christ's victory and also of Christian Baptism.
2. The moral sense. The events reported in Scripture ought to lead us to act justly. As St. Paul says, they were written "for our instruction".
3. The anagogical sense (Greek: anagoge, "leading"). We can view realities and events in terms of their eternal significance, leading us toward our true homeland: thus the Church on earth is a sign of the heavenly Jerusalem.
118 A medieval couplet summarizes the significance of the four senses:
The Letter speaks of deeds; Allegory to faith;
The Moral how to act; Anagogy our destiny.
Beginning with the literal sense.
The literal sense of scripture is just a fancy title for how we read every piece of literature. We read the literature as the author intended. Some authors write poems, and we should read them and interpret them as poems. Other authors write history and they should be read as history. Parables are just that – parables. Mystical visions probably shouldn’t be read as history, they should be read as mystical visions. This is the literal sense, what did the author intend, and all the other senses of scripture should be based on the literal.
Next is the allegorical.
That is right St. Paul gives us an example of this in Galatians 4:22-31 For it is written that Abraham had two sons, one by a slave and one by a free woman. But the son of the slave was born according to the flesh, the son of the free woman through promise. Now this is an allegory: these women are two covenants. One is from Mount Sinai, bearing children for slavery; she is Hagar. Now Hagar is Mount Sinai in Arabia; she corresponds to the present Jerusalem, for she is in slavery with her children. But the Jerusalem above is free, and she is our mother. …Now we, brethren, like Isaac, are children of promise. But as at that time he who was born according to the flesh persecuted him who was born according to the Spirit, so it is now. But what does the scripture say? "Cast out the slave and her son; for the son of the slave shall not inherit with the son of the free woman." So, brethren, we are not children of the slave but of the free woman.
So St. Paul is saying that these real women who really lived on a deeper level represent two whole groups of people. Reading the OT with Christ in mind makes the Bible twice as thick and infinitely more beautiful.
What does the Bible say about the moral sense?
The moral sense really asks – what does this have to do with me? Again St. Paul gives us an answer:
1 Corinthians 10:1-11 I want you to know, brethren, that our fathers were all under the cloud, and all passed through the sea, and all were baptized into Moses in the cloud and in the sea, and all ate the same supernatural food and all drank the same supernatural drink. For they drank from the supernatural Rock which followed them, and the Rock was Christ. Nevertheless with most of them God was not pleased; for they were overthrown in the wilderness. Now these things are warnings for us, not to desire evil as they did. Do not be idolaters as some of them were; as it is written, "The people sat down to eat and drink and rose up to dance." We must not indulge in immorality as some of them did, and twenty-three thousand fell in a single day. We must not put the Lord to the test, as some of them did and were destroyed by serpents; nor grumble, as some of them did and were destroyed by the Destroyer. Now these things happened to them as a warning, but they were written down for our instruction, upon whom the end of the ages has come.
So St. Paul is saying – see how they messed up – don’t do that. The opposite is true as well with being virtuous and holy.
Finally how about the anagogical sense?
The anagogical sense says – what does this have to do with heaven?
Jerusalem, the city is seen in Scripture as a figure for heaven.
Psalm 122:1-9 I was glad when they said to me, "Let us go to the house of the LORD!" Our feet have been standing within your gates, O Jerusalem! Jerusalem, built as a city which is bound firmly together, to which the tribes go up, the tribes of the LORD, as was decreed for Israel, to give thanks to the name of the LORD. There thrones for judgment were set, the thrones of the house of David. Pray for the peace of Jerusalem! "May they prosper who love you! Peace be within your walls, and security within your towers!" For my brethren and companions' sake I will say, "Peace be within you!" For the sake of the house of the LORD our God, I will seek your good.
So while the original readers are praising the city of Jerusalem, we sing the Psalm to sing of our Heavenly Jerusalem.