A Faraday cage or box is an enclosure designed to keep electromagnetic fields either out or in. It is made of a conductive material and works by canceling out the charge of the field striking it.
The Faraday cage was invented by Michael Faraday, an English scientist who discovered many fundamental concepts of electromagnetism. Faraday's first "cage" was an entire room lined with metal foil. While this may seem similar in execution to the notorious tinfoil hat, tinfoil hats are actually ineffective at blocking electromagnetism because they are not fully enclosed. As hats, they are open at the bottom, and they actually serve to conduct electromagnetism into the body and amplify it. The ideal Faraday cage is a solid piece and contains no openings; however, mesh enclosures can also shield electromagnetic fields, if the wires of the mesh are very thick and close together.