Yellow Apples (not to be confused with Golden Apples) are rare drops from leaf blocks along with Green Apples. They have a 1/200 chance of falling from the tree and can work in most dimensions.
Yellow Apples heal 2 <3, but every third Yellow Apple you eat only heals 1 <3. The only dimension they don't work in is BlackDown. They could also be used to prank people into re-naming the apple and enchanting it with an Anvil so it glows, taking the person to BlackDown, letting then get weak, then giving them the apple and during their frustration, they get killed by the BlackDown mobs. It can also be used for Taming/Breeding Elephants.