For the uninitiated, ZOPA stands for zone of possible agreement. In a negotiation, this is the range of outcomes that the parties should be willing to agree to.
Most people instinctively want to "win" as much value for themselves as possible (i.e. a deal on the edge of the ZOPA), but that intuition is both greedy and short-sighted. If there's a high probability that you'll negotiate again, you're likely better off trying to find a deal that shares value (i.e. a deal in the middle of the ZOPA), so that they'll be willing to do the same for you in the future. If your counter-parties later find out that you haven't, they'll likely be less willing to entertain future deals and may even compromise their own interests just to spite you.
The best deals make everyone feel like a winner.