This means that when Canada's terms of trade have improved – that is, when Canadians receive higher prices for exports and pay lower prices for imports – the terms of trade for the U.S. economy have tended to deteriorate. And the situation has been the reverse when Canada's terms of trade have deteriorated.
What this negative correlation in the terms of trade tells us is that even though the two economies are integrated, they have very different structures that, quite naturally, respond in different ways to certain shocks. Consider both the Asian crisis of 1997-98 and the events of the past five years or so, as China and India have risen to economic prominence.