What did James Polk mean when he declared "Fifty-four forty or fight!" in the 1844 presidential campaign? Did he follow through with this campaign promise?

He meant that under his administration, the US would accept a boundary between the US and British portions of the Oregon Territory no further south than the fifty-four forty line of latitude, or he would go to war with Britain. He did not deliver on this promise as the US/Britain boundary in the Oregon Territory was set further south than the fifty-four forty line without a war between the two nations.