Jesus Christ is the incarnation of God, who became a man in order to die on the cross so that through His death and resurrection we might have eternal life. Now, we know this from New Testament passages. But phrases like “in accordance with the Scriptures” are repeated a great many times in the New Testament to show how the life of Jesus was a fulfilment of the Old Testament. For example...
For I delivered to you as of first importance what I also received: that Christ died for our sins in accordance with the Scriptures, that he was buried, that he was raised on the third day in accordance with the Scriptures (1 Corinthians 15:3-4)
The “Scriptures” here must refer to the Old Testament. So we are right to ask where, exactly, does the Old Testament predict these things? Let's look at one Old Testament prophecy by way of example...
For dogs encompass me; a company of evildoers encircles me; they have pierced my hands and feet— I can count all my bones— they stare and gloat over me; they divide my garments among them, and for my clothing they cast lots. (Psalm 22:16-18)
Well, we know from the gospels that Jesus was crucified, which meant that a company of evildoers encircled Him and pierced his hands and feet. The gospels also record how the Roman soldiers cast lots (like playing a game of dice), where Jesus' tunic was the prize (John 19:23-24). This is just one example where the gospel writers often cite Old Testament passages which are fulfilled in the life of Christ. This prophecy seems very straight forward, but this Psalm was written by king David, and it's written in the first person. It really seems to be a Psalm which David writes about himself. So we do wonder whether it's supposed to be predictive, as we would expect a prediction about Christ to read something like “A company of evildoers will encircle him, and they will pierce his hands and feet...” Well, this is a kind of prophecy called “foreshadowing”, in which the actual events in history (like something which happened to king David) are themselves predictive of future events which happen to Christ. David wasn't crucified, but he went through some kind of experience which made him write as he did. God uses actual historical events as predictions to demonstrate His sovereignty over all history. This kind of prophecy seems strange to us at first, but once we begin to see it in Scripture we begin to recognise the great wisdom of God in doing things this way, because we see all the more how everything in this world is subject to Him, and all things happen according to His will and purpose.
Sometimes there are far more direct predictions of Jesus, and sometimes prophets saw visions, and what they saw were symbolic and somewhat cryptic. Often God's intention was not to show the people at the time clearly what they ought to expect, but rather so that when those things came to pass, those who saw them come to pass would understand. And that they would understand because they knew the Scriptures and could see them becoming clear. This is what it would have been like for those living at the time of Christ. As they saw the events of His life unfold, Old Testament Scriptures would be recognized as prophetic of these events, and this would authenticate that Jesus was, in fact, the Saviour.
But that doesn't necessarily mean that the Old Testament contained passages which had no meaning until their predictions came to pass. We also find passages in the Old Testament that have a “double fulfilment”. For example, in Isaiah we find a message given to king Ahaz that “a virgin shall conceive and bear a son, and shall call his name Immanuel”. This was to be a sign to Ahaz at that time to confirm God's word to Ahaz regarding the attack of their enemies. At that time, it may have meant that a woman who was that day a virgin would shortly announce that she was pregnant and the child's name will be Immanuel. That would have been the sign to Ahaz. But this same passage most certainly refers to the virgin birth of Christ later in history (Matthew 1:23). The name “Immanuel” means “God with us”, which is precisely who Christ is; the incarnation of God quite literally with us.
This is how Old Testament prophecy often works. And all of this is to show the absolute power and authority and sovereignty of God who is in command of all things. The more we consider the marvellous ingenuity of God's Word, the more we are amazed by His infinite wisdom.
For Jews demand signs and Greeks seek wisdom, but we preach Christ crucified, a stumbling block to Jews and folly to Gentiles, but to those who are called, both Jews and Greeks, Christ the power of God and the wisdom of God. For the foolishness of God is wiser than men, and the weakness of God is stronger than men. (1 Corinthians 1:22-25)