Description of the border between Canada and the United States meant to symbolize the harmony and mutual cooperation of the Canada-US relationship.
Whether the description remains accurate, in a post-9/11 era of fortified border crossings is debatable.
Nonetheless, the phrase is a reliable, if hackneyed, standby whenever: an American politician addresses a Canadian audience; or the Canadian government wants fair treatment on a contentious trade issue, such as Soft Wood Lumber or the Keystone Pipeline.
Like most expressions of Canada/US relations, it has zero political currency in the US itself.
Image Source: Wikimedia Commons