From the introduction:
Imagine that you're standing on your favorite D&D world. All around you, there might be forests, swamps, mountains, bogs, ruins, cities, and more. Beneath your feet, there are countless labyrinthine tunnels and cave systems, perhaps even the Underdark. Whether you’re traveling along your favorite D&D world’s surface or in the caverns below, we have a good idea of how to run a game in that environment.
Now what we if we look up? Sure, there might be a cloud city, but I mean what if we look way up? What if we look up at a star and wonder what’s up there? How do we go there? You might try flying, but you don't make it far before the spell fails and exhaustion takes over. Perhaps an airship? How does your airship fly when the air of the upper atmosphere begins to thin? Is this some insuperable barrier to exploration?
Absolutely not.
Wildjammer is a setting neutral rule-set inspired by the D&D campaign setting Spelljammer, created by Jeff Grubb and published by TSR, Inc. in 1989 as the flagship setting for D&D 2nd edition. Spelljammer gave us the means to bring our adventures up into space without losing any of the essential fantasy of D&D. It envisioned a universe structured in such a way that all D&D settings could be navigable without the rules or unique character of one setting hampering another. Your character from Faerun (Forgotten Realms) could travel into space and find Oerth (Greyhawk) or Krynn (Dragonlance), or any other setting the DM decides to add. The goal of this Wildjammer is much the same: to offer a rule-set that can be used in a D&D 5th edition game to unite different settings and do so in a way that doesn’t hamper the essential fantasy flavor of D&D.