A Faraday cage was an enclosure made of conductive materials, which could block electromagnetic fields.
The Drum contained a Faraday cage that was also lined with lead to shield against radiation. The crew of the base sheltered inside the cage when the ghosts appeared, as the ghosts could not penetrate it. Later, the Twelfth Doctor organised the crew to trap the ghosts inside the cage. The ghosts later escaped, but the Doctor used a hologram of himself with a recording of the call of the Fisher King to lure the ghosts back inside and trap them. Once they were inside, he contacted UNIT to deal with them. According to the Doctor, UNIT will cut the Faraday cage out of the base and eject it into space. Once far enough away from the Earth's magnetic field, the ghosts inside the cage will dissipate. (TV: Under The Lake, Before the Flood)