"fifty-four forty or fight! " which explains the significance of the motto in relationship to the movement for westward expansion?

The “Fifty-four Forty” raises to a line of latitude, an east-west running line marking a north-south border. This specific line was the anticipated national boundary between the mounting United States’ Oregon territory and Canada. During the 1844 presidential campaign, the Democratic Party declared that the US could claim all land in the Oregon Country north to the 54-40 line or parallel. The British disputed claims north of the 42nd parallel. 54-40 or fight turn out to be the slogan of the expansionists. Subsequent to the election, Democratic Pres. James Polk finally led the US to decide to a concession border at the 49th parallel where the northern boundary survives.

it proved indian land claims were unjustified.