(covers information from several alternate timelines)
"That wreckage belonged to the Terran Discovery."
The ship was launched from the Tranquility Base shipyard. Its motto was "Facile intelligere semel invento sunt omnia quae vero sunt, sed quae sunt difficilis quoestio est deprehendere." Translated from Latin, that was "All things can be understood once they are discovered; the point is to discover them," which was the motto of the prime universe's USS Discovery. (DIS: "Despite Yourself", dedication plaque)
In late 2256, the vessel was under the command of Captain Sylvia Tilly, who rose to the position after killing the previous captain. The same year, it was theorized by the crew of the USS Discovery that the vessel had crossed into the prime universe by the same spore drive malfunction that had stranded the USS Discovery in the mirror universe. As a result, Captain Lorca ordered his crew to modify all of the ship's systems and visual outlook to emulate the ISS Discovery. (DIS: "Despite Yourself")
The ISS Discovery, upon arriving in the prime universe, was destroyed by Klingons in a war against the Federation. The wreckage was mistaken for that of the USS Discovery by Starfleet, leading to the latter being declared KIA. It was nine months before Starfleet learned what the debris actually was. (DIS: "The War Without, The War Within")