What is Spelljammer?

Some background info:

In a D&D game, adventures can unfold in any corner of the multiverse, not just in the dungeons and wildernesses of the Material Plane but also on other planes of existence, including what celestial navigators refer to as Wildspace. When you stand on a Material Plane world and look up at the night sky, what you are seeing is Wildspace and, beyond that, the Astral Sea. Exploring these realms and the worlds they surround is the crux of a Spelljammer campaign.

Much in the way that oars and strong winds enable travel by sea on terrestrial worlds, magic items called spelljamming helms are used to propel and steer ships through Wildspace and the Astral Sea. A spelljamming helm customarily takes the form of an ornate chair in which the ship pilot sits.

Spelljamming: Spelljamming is the act of using a spelljamming helm (think throne-like chair) to propel and maneuver a ship. The individual that operates the helm is called a spelljammer (must be a caster and must be attuned to the helm to operate).

Wild space: a system of planets (think of like our galaxy) ... these are pockets within the larger astral sea. though unlike the astral sea, a wild space system does not have oxygen and functions like real life space (unless stated otherwise).

Air envelopes: When a creature or an object leaves a planet’s atmosphere and enters Wildspace, an envelope of breathable air forms around it and lasts until that air is depleted. this varies by creature size (small/medium creature will get 5ft cube). spelljamming vessels will have one much larger. this air can deplete and get foul, but it can also get replenished. it can also be manipulated by larger vessels and planets.

Gravity planes: In Wildspace and on the Astral Plane, gravity is an accommodating force, in that the direction of its effect seems to be “that which is most convenient.” for vessels gravity doesn’t radiate from a point but from a plane that cuts horizontally through the object, acting two-directional: a creature can stand upright on the bottom of a ship’s hull "upside down" and move around naturally. it can also be manipulated by other vessels.

The astral sea: you don't need to breathe air, eat/drink, you don't age, and you can propell yourself forward with thought alone. however, you only know the most direct route and not of any dangers that may lurk along the way. the location must be in the astral sea (an outpost) or a wildspace system.