Negotiation -- that give-and-take act of horse-trading. The idea is that you have something I want, I have something you want, and through an iterative process of offers and counteroffers, we arrive at a mutually acceptable agreement.
In real estate, the seller has a house, the buyer (presumably) has money or the ability to borrow some of that stuff, and each wants to swap. Let the games begin!
Now, during much of the past decade, agents didn't have to think too hard about the nuances of the negotiation process, thanks to "stated income" loans and a demand for housing that was, shall we say, a bit off-kilter.