Van Eck phreaking is the process of eavesdropping on the contents of a CRT or LCD display by detecting its electromagnetic emissions. It is named after Dutch computer researcher Wim van Eck, who in 1985 published the first paper on it.
Information that drives the video display takes the form of high frequency electrical signals. These oscillating electric currents create electromagnetic radiation in the RF range. These radio emissions are correlated to the video image being displayed, so, in theory, they can be used to recover the displayed image.