Short answers, pulled from the story.
The leidang was a coastal fleet levy that obligated free farmers in medieval Scandinavian kingdoms to build, equip, and crew ships for defense or seasonal expeditions. Administrative districts called skipreide were each responsible for supplying one ship, fully provisioned for two to three months.
The Icelandic sagas credit King Haakon I of Norway, known as Haakon the Good, with founding the leidang in the tenth century. The leiðangr of Norway is first mentioned by name in 985 AD in Skaldic court poetry praising Jarl Haakon of Western Norway and his son Erik.
Each skipreide was a coastal administrative district collectively responsible for building, maintaining, and crewing one defense ship. By the 1200s each skipreide comprised roughly 160 farms, and the district's leader, the styrimaðr or steersman, served as the ship's captain.
The Norwegian Older Law of the Gulating required every man to carry at minimum an axe or sword plus spear and shield, and each rowbench of two men to supply a bow and 24 arrows. Wealthier freemen in the twelfth and thirteenth centuries were expected to bring a helmet, mail hauberk, shield, spear, and sword.
By the twelfth to thirteenth centuries the leidang converted in many areas from a ship levy into a tax paid by all free farmers, which persisted in some parts of Scandinavia until the nineteenth century. Around 1660, the skipreide districts were converted into tinglags, civil court districts incorporating community or city courts.
The Anglo-Saxon equivalent was the fyrd, a district militia dating back to at least the seventh century. Alfred the Great reformed the fyrd and combined it with fortified burhs and a fleet to counter Viking raids; Henry I later called it out as an army of all England in both 1101 and 1102.