The Bible is such a large book that the vast majority of Christians, at least in the Western world in which we are privileged enough to have it, are too lazy to actually read it. It is a collection of 66 books, written over a span of many centuries. It records 4,000 years worth of history, as well as preparing its readers for the next 2,000 years (at least), and gives us instructions on how to live our lives. It contains hymns and poems, commandments and doctrines, sermons and parables, and many great things.
The culmination of all of these things is one spectacular message, commonly known as "The Gospel". The Gospel literally means "the good news", and it is the central theme of this giant book. Amazingly, while the Bible is big, the Gospel is so easy to grasp, even a child can understand it. As one theologian said, the Bible is shallow enough for a child not to drown, yet deep enough for an elephant to swim.
The Bible begins at the very beginning of time, when God created the heavens, the Earth, and all that is in them. This included two humans, who were perfect, and more importantly, had direct and flawless fellowship with God. However, being tempted by the devil, they disobeyed Him, resulting in the very punishment God had promised them ahead of time: Death.
But that wasn't the end of it, for God had already planned the way of redemption ahead of time. Immediately after sin, God promised a Savior. In due time, this promise was fulfilled, as the Savior was born as a helpless baby boy, through the womb of a virgin woman. This child, named Jesus, grew up, living a perfect, sinless life from beginning to end.
At the end, Jesus suffered a brutal death, specifically capital punishment by way of crucifixion. But if He was without sin, why was He punished? Simply because we, who are not without sin, deserve the full wrath of God. The punishment we deserve, He received, allowing us to receive the reward He deserves. The fancy name for this doctrine is "Penal Subtitutionary Atonement".
This puts humans between two "Adams". The first Adam, a rebel, brought sin and death into the world, and we, his descendants, inherit his curse, just as we imitate, and often exceed, his rebellion. The second Adam, God's own Son, paid the price owed to us, buying us an eternal inheritance in the Kingdom of Heaven. This is received by faith. All who confess Jesus as Lord, and believe in their hearts that God raised Him from the dead, will be raised with Him in due time. Thus, Scripture says, as in Adam all die, so in Christ all shall be made alive.