What did james polk mean when he declared "fifty-four forty or fight! " in the 1844 presidential campaign?

Polk's "Fifty-four forty or fight!" slogan was in reference to the dispute arising between the United Kingdom and the US surrounding the Oregon Territory border. The Democratic Party, of which Polk was a member, took a particularly aggressive stance during the 1844 elections; in essence, the slogan was a threat to the United Kingdom: either the US gets all of the territory up to the 54,40 parallel or there will be war. The British, on the other hand, wanted the boundary along the Columbia River, encompassing the area of present-day Seattle. 

The Fifty-Four, Forty parallel exists at the southern tip of Alaska and encompasses much of modern day British Columbia. In the end, both nations settled on the 49th Parallel North, the boundary as it exists between the US and Canada today.