What is the meaning of "Fifty-Four Forty or Fight?"

This phrase was a slogan for people who wanted to expand
the United States far to the north in the Oregon
Territory.

According to a treaty that was signed in 1818,
the US and Great Britain were to jointly occupy the Oregon Territory, from what is now
the southern border of Oregon to the line at 54 degrees 40 minutes North
latitude.

In the 1830s and 1840s, Americans started to want
to expand.  The expansionists wanted the Oregon Territory to belong solely to the US. 
The slogan you mention was used by the most extreme of the expansionists.  The meaning
of the slogan was that the US should go to war with Britain unless Britain gave the US
the entire Oregon Territory.

This issue ended with the
boundary being set at the 49th parallel.