The Apple barrel is an item located in the two breweries, one in Keldagrim and one in Port Phasmatys used to crush cooking apples to make Cider. It is used by placing 4 cooking apples inside it with an empty bucket in your inventory, creating a bucket of Apple mush.
It is also a quest item used in the Mourning's Ends Part I quest. To make this Apple Barrel, pick up a barrel from a spawn north of West Ardougne, just west of King Lathas' Training Camp during the Mourning's Ends Part I quest. Use the barrel on the pile of rotten apples located in the same place. You have to have already talked to Elena in West Ardougne to fill the barrel with rotten apples. Use the apple barrel on the apple barrel machine located in the same place as the rotten apples. You will have a barrel of apple mush. The only use the apple barrel has after the quest is to make apple mush, which in turn is used to make cider. Apple barrels may also be found in breweries. Note that in the quest Mourning's End Part I, the item will not be called Apple barrel, it will be called Rotten apples.