[Conversation] is a process of two people understanding each other. Thus it is a characteristic of every true conversation that each opens himself to the other person, truly accepts his point of view as worthy of consideration and gets inside the other to such an extent that he understands not a particular individual, but what he says. The thing that has to be grasped is the objective rightness or otherwise of his opinion, so that they can agree with each other on a subject.
In a conversation, Gadamer argues, we have what is called a "horizon of understanding". That is
'the range of vision that includes everything that can be see from a particular vantage point' (ibid: 143) When we attempt to understand what someone is saying, we often times have to put ourselves "in their shoes" so to speak. We have to gather an understanding of what they are going through and their "horizons" so that we can understand why they say something.
This same concept also applies to understanding the Bible.
When we try to understand specific biblical passages, it's often helpful (or even necessary) to try to understand the culture that the passages were written in. We have to try to understand the "horizons" of both the authors and the culture of the time to better understand how the original text was meant.
A (probably poor) example of this is understanding specific aspects of the crucifixion. For example, Why was the sign "King of the Jews" hung above Jesus' head? If we take a look at the culture at the time, we see that crucifixions were common methods for the Romans to kill criminals. They hung the reason for their crucifixion above their heads to let others know what crimes to avoid.
Without the cultural understanding of the times that the Bible was written, there would be a lot of things that we would fail to understand. Fully researching the culture helps us to gain the "horizon of understanding" of the authors and events of the Bible so that we can better understand the Bible itself.