A long-haul flight is one that covers a very long distance — usually many hours, often between continents. Taipei to New York or London is long-haul; Taipei to Tokyo or Hong Kong is not (that is short-haul, or a shorter regional flight). Airlines, airports, and travel ads use this pair all the time: long-haul / short-haul. You also hear it about trucks and cargo that travel far. Here haul means the length of the trip, not “pulling a bag.” There is another English phrase, in the long haul, meaning “over a long period of years,” but that is a different use; this sense is about distance. In speech it is everyday in travel talk, not fancy.