When the game was invented and sold in a little woodgrain box, the author told us a required supplement was an Avalon Hill game called Outdoor Survival. This was a wilderness survival game that consisted of a hexagonal map system that players would travel around, trying to find their way back to civilization, all the while trying not to die of thirst or get eaten by bears. This game map was used as the first wilderness "hex-crawl" for what eventually became D&D. Later, Judges Guild took this to a whole new level with the Wilderlands series. For many years, hex crawling was just the way the game was played. This series brings that back, or supplements existing games that use that system of travel.
What a hex crawl is, literally, is a wilderness sandbox of areas, encounters and villages that players travel around in. It provides no story line, just hundreds of story hooks and possibilities. An example of what this looks like that I published a few years ago can be found at: http://www.necromancergames.com/pdf/lenap/lenap.pdf
These books provide a sub-setting in your own campaign world. They populate the world, and allow you to let your players explore that world, rather than just "travel 20 days" to the dungeon. Written by John Stater of NOD fame, each of these supplements details an area with a specific theme. Monster and NPC statistics are provided for each encounter area detailed.
When the northern men came into this land they thought they had discovered a place of limitless possibilities - sparkling streams, fertile valleys and endless acres of rich timber. They didn't realize they would have to contend with giant beasts, rapacious lycanthropes and the mysterious black arcs that visited the land from beyond the Black Water. Strike out into this land of dire wolves and giant badgers - bring news and trade to the isolated colonies of the northern men or fight through the lands of the wolfmen to find the palace of the mythical Queen of the Winter Wind.