How does aluminium foil help in hiding metal from a metal detector?

[aluminiumsheet]

By forming a Faraday cage around what's inside and the detector see only the "cage" of aluminium foil. The explanation is the same for every metal, not only aluminium. But keep in mind that the "blinding" method depends of some things e.g. the frequency of the detector, energy, form of the detector coil.

[haomeialu]

Aluminum foil will block the x-rays used to x-ray packages going through the Post Office. Notably, it passes through large luggage studs, those being quite thick, through metal springs. Aluminium is non ferrous metal that is not detectable with most metal detector