The NW Passage is basically the shipping route north of the American continent. It was discovered finally by the Norwegian Roald Amundsen - the dude who was the first to reach the South Pole -, and it is very significant because... well it isn't as most of the time it's unsailable as it's frozen solid. In the past few years it's importance grew as with the global warming it stands open most of the time and certain ships just cannot into Panama canal so they have to travel south of South America and for some destination a northern route would be more advantageous.
But back in the 19th century it was completely useless but they kept trying and hoping it's just seasonally frozen.