The phrase "airtight tin" refers to a container made of metal that keeps air from getting inside. It is tightly sealed, preventing any air from entering or escaping. Full definition
Price: Premium Matcha Powder 30 grams for $ 29 Package: Airtight tin Ships: * Please note that this matcha is no longer sold online. (theteacupoflife.com)
6 Put the cake, still in its tin, on a wire cooling rack — it will hold its heat and take a while to cool; once cool, take it out of the tin and, if you don't want to eat it immediately (like any fruit cake it has a long life), wrap it in baking parchment or greaseproof paper then in foil and store in a cake or other airtight tin. (styleathome.com)
My friend made them but hers only stayed crispy the day she made them (although she put them in a food bag rather than airtight tin) (deliciouslyella.com)