An Acronym for Cathode Ray Tube.
The main component of traditional monitors and TV screens. Inside the tube there is vacuum, through which a high voltage accelerator in the back of the tube fires a stream of electrons at the phosphor coating on the inside of the front of the tube. The phosphor particles hit by the electrons momentarily emit light. An electrically induced magnetic field bends the trajectory of the ray in varying directions so that it covers over the entire phosphor surface. All along, the intensity of the ray is constantly adjusted to match the desired brightness of the currently targetted pixel. RGB displays can be created by using different phosphor compounds that emit light at different wavelengths for adjacent pixels. The main challenge faced when designing CRTs is focussing the ray tightly so that it illuminates only the desired pixels and doesn't "spill over" any energy to adjacent ones. The more focussed the ray, the sharper the image and the higher its contrast.
By scanning the entire area quickly enough, the ray can start over soon enough that previously illuminated phosphor particles can be fired electrons at soon as (or just before) they go completely dark. This way, an illusion of a steady image can be created despite the ray lighting up only tiny areas of the screen at a time. The frequency of full screen scans is called the refresh rate, and measured in Hz. In order for the brain to disregard the flicker of refreshing the image, the refresh rate has to exceed about 70Hz. However, it still perceivable and keeps the image "jittery" until about 100Hz are exceed, at which point it becomes completely impercetible.
You can try this by bringing up a bright image, like a fullscreen white image, and then looking at point away from the screen such that it is in your peripheral vision a fair bit off the center. This area of the human and the image perception of the brain are particularly receptive for motion and will often detect CRT flicker even if the image seems absolutely rock solid and steady when in the center of your vision.