Short answers, pulled from the story.
Spelljammer is a campaign setting originally published for Advanced Dungeons & Dragons second edition that places the game's usual fantasy adventures inside a fantastical outer space environment. Ships in the setting typically look like galleons, animals, birds, or fish rather than science-fiction spacecraft.
Jeff Grubb created Spelljammer, which TSR published starting in 1989 as the first of a new wave of campaign settings. Games historian Shannon Appelcline called the setting innovative and popular in his 2011 book Designers & Dragons.
A spelljamming helm is the device that lets a ship travel through interplanetary and interstellar space by converting a spellcaster's magical energy into motive power. The mysterious race known as the Arcane are the sole known manufacturers and distributors of these helms.
The Spelljammer is a legendary living ship shaped like a gigantic manta ray that carries an entire city across its back. It absorbs heat and light instead of eating, normally wanders without a captain, and only one exists at any given time.
Wizards of the Coast released Spelljammer: Adventures in Space on the 16th of August 2022, updating the setting for Dungeons & Dragons 5th edition. The boxed set included a Dungeon Master's screen, a poster map, and three hardcover books.
TSR ended the AD&D Spelljammer line in 1993 after Space Lairs and The Astromundi Cluster shipped as its final releases. TSR replaced it that same year with the new Planescape setting, built around the City of Sigil.