Cathode ray tube
A cathode ray tube (usually CRT for short) is a now-obsolete display technology that was the mainstay of visual output devices for a century, until displays consisting of arrays of LEDs, etc, became available economically.
It consisted of a large vacuum tube, with a large face covered with a phospor, a phosphorescent powder. An electrode at the base of the neck (back) of the tube produced a beam of electrons, which are shot into the phosphor; where the beam hits, the electrons hitting the phosphor produce a spot of light.
The intensity of the beam could be modulated by a control signal, from fully off to full brightness. The beam could be scanned across the face by means of capacitative or magnetic control grids, through which the electron beam was sent.