Apparently it means that the apple once was on the plate, but now it's no longer there. It may be elsewhere, or it may have been eaten. As I explained in my other post, this sense of for appears to be inspired by German fort (cognate of English forth), and not related to the more obvious English for/fore (cognates of German für/vor). And not related to Italian fuori (meaning outside), either.
It could mean that the apple was once on the plate, and is there no longer, but the Esperanto "for" can also mean "distant from" or "away from". An example given in Plena Ilustrita Vortaro is "Tio estis for de mia intenco". ("That was far from my intention").
I think what's going on here in Esperanto may be this: Zamenhof, looking for ways to dramatically reduce the number of words to learn, used a German idiom here. In German, the preposition fort (cognate of English forth) is used in the sense gone. The problem is that the t in fort (corresponding to the th in forth) isn't meaningless, and it's confusing to give up the distinction between English fore / German vor on one hand and English forth / German fort on the other, merging them into a single Esperanto word for.
Then, of course, it's entirely possible that Zamenhof took this overloading of for from some other language that I don't speak, such as Polish.