The scriptures principally teach what man is to believe concerning God, and what duty God requires of man.
Okay, so I already knew this one. It's good to think that I have either worked out the right doctrine through my own study of God's word or that I have sat under some good teaching (church sermons, BSF, small group Bible studies, night lectures at theol college...) which has taught me the great truths of the Christian faith articulately enough that they have been included within my own understanding of the faith (which, I'll freely admit, still has a lot of room to grow).
The Bible includes a whole lot of different genres of writing, by a number of authors who lived over a period of several thousand years and wrote with different immediate audiences in mind. Despite this apparent diversity in content, it is pretty clear to anyone who has attempted to summarise the Bible's message into one sentence (as I recall being asked to do 5 or so years ago in a small group Bible study) that the Bible is all about who God is (aka theology, or knowledge of God; the sound basis of our faith and belief) and also about how we should live in light of who God is (aka Christian duty, morality, righteousness, holy living, good works which God has prepared for us as per Ephesians 2:10 & 2 Timothy 3:17, working out one's salvation with fear and trembling as per Philippians 2:12).
Both of these elements are necessary for us as Christians. It is not enough to know intellectually who God is, we must act rightly in response to this knowledge. James wrote (James 2:19, 22-23), "You believe that there is one God. Good! Even the demons believe that - and shudder. ... You see that [Abraham's] faith and his actions were working together, and his faith was made complete by what he did. And the scripture was fulfilled that says, 'Abraham believed God, and it was credited to him as righteousness,' and he was called God's friend." Jesus used a parable of two sons to explain this inescapable duality (Matthew 21:28-32). It is not enough to recognise God's authority with words, one must also act in accordance with what He commands with His authority.
Likewise, morally upright behaviour that is not done in full knowledge of God and submission to Him is insufficient, in that it does not further the chief end of man in glorifying God and enjoying Him. This was the attitude of the Pharisees who became so caught up in obeying their interpretations of God's commands that they forgot who they were meant to be obeying and refused to recognise God's Messiah when He came. Jesus criticised these people in his parable of the tenants (which follows the former parable, in Matthew 21:33-45). In this parable, those who follow the law without recognising the lawgiver are told clearly (Matthew 21:43) that "the kingdom of God will be taken away from you and given to a people who will produce its fruit."
Obviously there are bits of the Bible that do not add to this core message of doctrine and duty; that's why this question asks about what the Scriptures principally teach. There are more and less informative parts of the Bible, and some parts of the Bible are actually more important than others, in line with how they increase one's understanding of either of these two topics. However no part of the Bible is unfruitful or without its purpose. "All Scripture is God-breathed and is useful for teaching, rebuking, correcting and training in righteousness," as Paul wrote in 2 Timothy 3:16.
We read the Bible so that we might know God and learn to do His will.