Apples are food that can be gathered from Apple trees, by right-clicking on it and choosing Nature's lore -> Gather apples.
Depending on the quality, the state of the Apple tree and your Nature's_lore skill level you can gather 0 - 7 Apples (15 from self-planted Apple trees). Note that the number of apples gathered in one pass may not be all the apples on the tree. A typical case is getting 6 on the first try, and 1 on the second. (The lower the Nature's_lore skill is, the more tries are needed to gather all apples.)
The quality of gathered apples depends on the players Nature's_lore level as well as the Apple tree's quality.