Original: The Prelude and Liebestod is a concert version of the overture and Isolde's Act 3 aria, "Mild und leise". The arrangement was by Wagner himself, and it was first performed in 1862, several years before the premiere of the complete opera in 1865.
Simplified: The Prelude and Liebestod is a concert version of the overture and Isolde's Act 3 aria, arranged by Wagner, which was first performed in 1862, before the first performance of the opera itself in 1865.