can anyone explain this sentence?

A bargaining chip is something you can offer someone which might motivate them to concede something you want. Senator White offered his vote on the farm subsidy bill as a bargaining chip to get Representative Brown to support the House version of his fuel surtax bill.

As for the rest of it, you haven't provided nearly enough context for anyone to give you a meaningful answer. We don't know what Kissinger's bargaining chip really was, nor how it related to restraint on Moscow's part. It seems apparent that Kissinger's bargaining chip was something he could use in negotiations with the members of the United States Congress, but who the hell knows for sure? You haven't given us enough context.