The name means Iron Bear, though misspelled. Jarnbjorn was originally forged by the dwarves of Asgard as Thor's original training weapon.
Following a battle with Apocalypse, Thor was tricked by Kang the Conqueror, who was disguised as Loki, to augment the blade with the power to pierce Celestial armor. During Thor's rematch with Apocalypse, Thor was able to destroy the mutant's armor and destroy his ship. Jarnbjorn was then stolen by Kang, and in turn the Apocalypse Twins, who now use it to murder a Celestial and collect their Life/Death Seeds.