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.
In ancient times the Pirate Coast was home to a great civilization of stone giants. The stone giants maintained a wide-reaching kingdom of towns and villages. Despite their great size and power they relied on slaves taken from the primitive human tribes of the coast. In time, the stone giants grew lazy and decadent, and when humanoids poured into the coastal lands and gradually destroyed the stone giant kingdom. The stone giants were more powerful than the humanoids, but they were more numerous and they outbred the giants. Worse yet, the stone giants found themselves plagued by slave revolts. The humans of the coast founded their own kingdom on the island to the east of the coast and came to be called the Bucranians by the bull skulls they used as standards. More than a century ago, the roving pirates of the White Island discovered the wealth of the Bucranians and began raiding their villages and towns. When the White Islands sank, many settled along the Pirate Coast and started making deeper and deeper raids into Bucrania, burning villages and carting away slaves from the old and lethargic kingdom.