When a dot is drawn on a cathode ray tube, the spot lasts for a time that depends on the type of phosphor used in the tube. Another side effect is that the area around this dot is slightly changed in electrical charge. By measuring the charge at that spot you have a simple form of memory that lasts for a time depending on the type of phosphor used.
Developed at the University of Manchester in England, it provided the medium on which the first ever electronically stored-memory program was written. Tom Kilburn wrote this 17 line program to calculate the highest factor of a number.
The Williams tube was regarded as extremely unreliable, and touchy. Most working installations had to be "tuned" by hand. In contrast, delay line memory was slower, but quite reliable. This is why delay lines were used in most machines that were regarded as successful.