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 Gathering 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 Gathering skill is, the more tries are needed to gather all apples.)
The quality of gathered apples depends on the players Gathering level as well as the Apple tree's quality.