Yellow kryptonite is one of several different color variations of the rare mineral known as Kryptonite. Kryptonite is found in material relating to the Superman multimedia franchise and has been seen in many varities throughout the comics as well as television and movie adaptations. Yellow kryptonite first appeared in "The Conquest of Superman" story from Action Comics #277. The concept behind yellow kryptonite was developed by writer Bill Finger and artists Curt Swan and John Forte.
Yellow kryptonite is not actually true kryptonite. In pre-Crisis continuity, criminal mastermind Lex Luthor created yellow kryptonite as a means of scaring off Superman so that he could rob Fort Knox. Ultimately, the joke was on Luthor as the Superman he had managed to drive away was actually just one of his cleverly disguised robot decoys.
Another and more prominent version of his hue is actually gold kryptonite and has the ability to remove a Kryptonian's super powers.