Here, Lucy is using it to keep the cookie sheet clean -to protect the cookie sheet.

这里，露西用它保持烤盘干净——目的是保护烤盘。

So, the aluminum foil protects the sheet, and so the cookie sheet doesn't get dirty - the pan doesn't get dirty.

所以，铝箔纸保护了烤盘，烤盘不会脏——模具不会脏。