It means that if you read the OT, and NT, like you would a history text book for a history class you are doing it wrong. You need both OT and NT, they explain each other.
There are ways of reading the Bible and some are better than others.
It can be read as History but this is the lowest form. For instance, the first and second creation story are slightly different or the NT has Jesus saying the same things but in different locations. However, the Bible was the only historically accurate source of small points as well (the existence of Ur, the length it takes to mummify a person which is no where in Egyptian literature). Simple historical facts are of limited use to your soul.
The Bible can be read eschatologically. As a for-showing of what the end of times are going to be like.
It can be read as prophecy-fulfillment. OT prophet says X and in NT Jesus or Mary or the early Church does X.
It can be read Metaphorically. This is where the best stuff is. The really cool things.
For instance, the Davidic Kingdom. That it existed is neat but not what is most important. David made psalms that are prophetic of Christ and of the end of times. However, when you look at the works of the Davidic Kingdom things start to really open up. David was King and offered sacrifices, at his right hand sat his Mother who took supplications of the people to the King, and he had a Prime Minister that ruled in his absence who was given the symbol of authority of Keys. What does this sound like? Jesus as King and Priest whose enthroned Mother offers our intercessions and the Pope, with the keys, takes care of His kingdom while he is away!!!
It is not that you should not read the Bible for history. You should, there is much there that did happen, many neat facts. But, history is not the point of the Bible, only its backdrop, a narrative device.