Older CRTs (the big, heavy, glass tube-style TVs (basically everything pre-LCD and pre-plasma)) use a firehose approach to drawing images on the screen. Imagine three big, sweaty firemen spraying their three hoses back and forth along the back of a giant drive-in movie screen that's been painted with red, green or blue stripes. There's a grid in between the firemen and the screen, so that each fireman's hose will only spray one colour, the grid prevents their stream from hitting any other colour. They spray in sync: special signals tell them when to start again on the other side of the screen, and when to start at the top. For their entire working lives they zigzag right and left, from top to bottom, over and over and over.
Basically, that's how the old TVs worked: three electron guns sprayed electrons at different coloured phosphors, which would light up based on the intensity of the beam. It was never very accurate: sometimes the beams would drift out of alignment, failing components caused all kinds of undesired visual artifacts, and even the Earth's magnetic field would wreak havoc on the display.
For all these problems, they had some capabilities that the modern fixed-resolution LCDs and plasmas can't match. While modern displays have caught up or surpassed CRTs in many respects, in two specific criteria the old units are better than modern displays: image scaling and refresh speed. I'm going to focus on the former, for now.