The idea is pretty simple by most standards. In all deals and cases of human interaction, strive for an outcome where both parties leave better off than otherwise. I pay you money, you get a product. There are two problems in the way of this habit: 1. That most people, as they grow in an atmosphere of competition, think in Win-Lose terms. I bet you money, you win the bet, I get nothing. I lose, you win. Or Lose-Lose terms. If I'm going down, I'm taking you with me. 2. Win-Win is not applicable in all situations. When two teams are playing soccer, this is not an every-one wins situation and to make it such would destroy the purpose and fun of the game.
The key seems to always strive, but understand when it is not possible. In business, Win-Win should always be possible. In fact, Covey says there should, in most cases, be a Win-Win or No Deal situation. Either we both win, or there's no purpose to our transaction. I think the point is that most social exchanges in life are not competitions. They are deals.
Let me give you a quick example. When you go out to eat, do you feel you've gotten the food you've paid for? If not, do you still eat at that same place? The business always wins with your money, but you can feel you've won if you enjoy the product. It's the same from a business point of view, as well. If you want to make money, you want to please your customers and make them feel as if they've won something as well. Both parties leave with more than they've started with.
Finally, the next best outcome to Win-Win is compromise, a lesser form of the idea. Both parties bend to the other to meet in the middle. Haggling, as it were.
There's a deeper principal here, somewhere, that I've not grasped, yet. An idea that talking with someone can lead to a Win-Win situation. As if someone speaks to me and wants something, even if that something is just for me to listen. What do I get out of it? What's my time worth? What is your's? There's something in this habit I haven't figured out yet. More study may be needed.