A type is an Old Testament foreshadow of something usually brought to further light in the New Testament. Other words to help explain the concept could be pattern, model, template, preview, snapshot, puzzle piece, one part of a portrait, blueprint, or a prefigure.
An antitype is the foreshadowing in its more full state. It is the fulfillment, in a manner of speaking, of the type. It is the OT idea of it come into fruition, it is what the type was in some way pointing towards. The type may be thought of as the sign for a road but the antitype is more like the actual road with the pavement and all that. The analogy is limited but I want to give a variety of perspectives on the relationship between type and antitype in hopes to make it more clear as to just what the theological concept entails.
I am using type in a broad sense here, as it is easy to become extremely precise in what one calls a type and then use other labels to describe the other OT foreshadowings of Christ. But here I will use type as a relatively sweeping term in order to show three examples of types from the Old Testament which anticipate Christ in John.
1. Jesus is the Final Passover Lamb. In the Old Testament, a lamb was slain every Passover to mark the Exodus. More specifically, it recollected how the Israelites escaped the wrath of the Destroying One in the Tenth Plague by the blood of the Lamb. This is how they were rescued, and we are also rescued by the blood of the Lamb. Also, the Passover Lamb would not have any of its bones broken and neither did Jesus (Ex. 12:46; Num. 9:12; Ps. 34:20).
2. Jesus is the True Manna. In John 6, Jesus explained the Old Testament points to Him. He told the people he was the true bread come down from heaven and if people ate of his flesh they would love forever, whereas in the wilderness people had to eat again and again and yet they still eventually died. Jesus is the antitype of the manna in the Old Testament.
3. Jesus is the New Moses. In the Old Testament, Moses delivered the Israelites from bondage. Jesus saves his people from their sins with his actions. John 1:17 even compares Moses to Jesus: “For the law was given through Moses; grace and truth came through Jesus Christ.” Jesus gives another grace, like Moses gave, and it is built upon the “work” of Moses but it is the completion of it – the ultimate step in God’s grace program, if you will (John 1:16). And just as Moses explained YHWH to the people, Jesus exegetes the Father!
Other parts of the NT explain that since Moses’ revelation was rejected (Luke 16:29–31), Jesus’ words were rejected. Yet in John 12:41 Jesus says that Isaiah had written about Him; this again shows that Jesus understood the OT to be full of types and prophecies about him. This does not mean every single thing in the OT is a type but simply that the OT does indeed point to Jesus.