'zero-sum game' is a type of negotiating strategy. It basically means that one side/party in a negotiation wins (or benefits more than it should) and the other side/party loses (or benefits less than it should) in a deal or agreement.
Normally, this type of negotiating strategy is very common in one-off negotiations, where there is little possibility of further deals (called 'follow on business') in the future. It is not commonly used in business-to-business negotiations.
The opposite strategy of 'zero sum game' is 'win-win'. With 'win-win', both the sides/parties in a negotiation benefit from the deal or agreement that is reached.
If a salesman sells a car to a customer for a lot more money than what the car is worth, then this is called a 'zero-sum-game' negotiation. The salesman is the only one of the two who is really benefiting from the deal.
To learn more vocabulary connected to negotiations, do a free online exercise on negotiation related vocabulary.