The "bit" is Transubstantiation, a key Miracle of the Catholic faith. Catholics believe that the Eucharist blessing, a ritual often performed in Churches on a Sunday, literally transforms the bread and wine - they aren't just symbols, they literally are now Christ's Flesh and Christ's Blood. Now, you may observe that, just as "An Oak Tree" looks, and in every way seems exactly like a glass of water, the bread and wine still looks and in every way seems exactly like bread and wine, but Catholicism teaches that nevertheless they are the Body and Blood of Jesus.

(Roman) Catholics who care will be told this is because all observable properties are just incidental whereas the substantial nature changed, just imperceptibly.

Now, the exact details will vary outside Roman Catholicism, but you should generally expect that either they're deliberately vague (Eastern Orthodox and Anglican traditions) or they have the same sort of dubious philosophical out, and that's what this artwork is about.

So, it's not really necessary to the "bit" that you insist it's actually An Oak Tree at customs. Catholicism works around this question (obviously it's not OK to import thousand year old flesh or blood without a very difference license from bread and wine) because the bread and wine are just bread and wine up until the ritual is performed, you could do the same by just deciding a different glass of water on a shelf is "An Oak Tree" to skip the customs check.