Looks like there may be a confusion of terms. In your subject line you say an "apple core". In your message you say you don't know where an "apple" came from. Those are wonderfully different things.
Apples come to people for making good posts. They're like a big thumbs up. Once you've been a good poster on the site long
enough you get the ability to give an apple a day to posts that you think are worthy.
Apple cores are the opposite and there aren't many of them. If you ever do get a core, the links Anne and Paul posted would help you figure it out.
It looks like you have only ever received apples (no cores) so keep up the good work!