what is "slack pack"?

BrianLe may give you a better answer, but my understanding is that "slack packing" consists of arranging matters so that you are not carrying all the necessary equipment to camp overnight as you hike, even though your hike is longer than one day. You can do this by having someone else haul your equipment and meet you at the end of the day, or by emerging from the trail at day's end and staying in a town, then continuing back on the trail the next morning.