It was one of the Luke Skywalker's lightsabers - the first lightsaber he constructed and the second one he owned.
This weapon's design was based on schematics Luke found in his mentor's hut on Tatooine. While the final product remained largely faithful to these designs, Luke simplified the weapon considerably, as this was his first attempt at constructing a lightsaber. For instance, he decided use only one crystal for his lightsaber, as opposed to the ideal three to allow for a fully adjustable blade. Luke was able to gather most of the parts he required to build his weapon from various vendors in the city of Mos Eisley, including a high-energy reflector cup, power cells and a focusing lens. However, Luke was unable to obtain the one vital component that would bring the whole project together: the lightsaber crystal itself.
In the years following the Galactic Empire's establishment in 19 BBY, great efforts were taken to expunge the last remaining resources of the fallen Jedi Order from the galaxy. One of these measures was the banning of all crystals capable of being used in the construction of lightsabers, and shutting down the sites that possessed these crystals. Possessing lightsabers was also outlawed with the exception of Palpatine himself, Vader, the Emperor's Shadow Guards, and the Emperor's Hands. Others who possessed lightsabers collected them as a trophies from fallen Jedi. Due to this, Luke was forced to fabricate his own green synthetic lightsaber crystal, utilizing a small blast furnace and a book of instructions.
For many days, Luke labored over the components, checking and then re-checking the instructions many times, taking no shortcuts. Over a month after he first began, the weapon still was not complete. Luke had to interrupt his work on the project while he and his friends mounted an ultimately unsuccessful rescue for Han Solo from bounty hunter Boba Fett's custody on the planet Gall. Returning to Tatooine, Luke resumed his work on the weapon, cutting and shaving the unrefined crystal he had created into a proper multi-faceted lightsaber jewel.
When Luke Skywalker first ignited the weapon, it functioned perfectly. The green blade emitted no heat, signifying that the internal superconductors were working properly, and it sheared through a wrist-thick rock spire with no difficulty. Luke also noted that the weapon possessed superior balance to his previous lightsaber. Possibly as a result of utilizing a synthetic crystal, the lightsaber made a distinctive snap-hiss sound when activated, which was a different sound to that emitted by his father's weapon.