From Halopedia, the Halo wiki
A Faraday cage or Faraday shield is an enclosure formed by conducting material, or by a mesh of such material. Such an enclosure blocks out external static electrical fields. Faraday cages are named after physicist Michael Faraday, who built one in 1836.
An external static electrical field will cause the electrical charges within the conducting material to redistribute themselves so as to cancel the field's effects in the cage's interior. This effect is used, for example, to protect electronic equipment from lightning strikes and other electrostatic discharges.
To a large degree, Faraday cages also shield the interior from external electromagnetic radiation if the conductor is thick enough and any holes are significantly smaller than the radiation's wavelength. For example, certain test procedures of electronic components or systems that require an environment devoid of electromagnetic interference may be conducted within a so-called screen room. These screen rooms are essentially labs or work areas that are completely enclosed by one or more layers of fine metal mesh or perforated sheet metal. The metal layers are connected to earth ground to dissipate any electric currents generated from the external electromagnetic fields, and thus block a large amount of the electromagnetic interference. This application of Faraday cages is explained under electromagnetic shielding.
The UNSC stealth cruiser Point of No Return is equipped with a secure, insulated room known informally as Odin's Eye or "the cage", which functions as a Faraday cage and is as such considered one of the most secure places in human territory. During the Onyx Conflict, thermal blankets were used to create a primitive Faraday cage around Dr. Catherine Halsey when she was working on a laptop, to avoid detection by the Onyx Sentinels' electromagnetic sensors.