Something that's fallen has dropped to the ground. You might collect fallen apples to make into cider or applesauce.
The most literal sense of something fallen is just what it sounds like — an object that's moved from a high place to a lower place, usually due to gravity. There are also some figurative meanings: a fallen soldier has died on the battlefield, and a fallen person has committed a moral sin or ruined his reputation. In the 17th century, fallen frequently meant "morally ruined," more often describing a women than a man.