Old televisions used vacuum tubes in their circuitry in a similar role to transistors in (more) modern electronics.

These were literally little glass bulbs with bits inside that heated up, glowed and did magical things with electrons. They had some number of pins on the bottom and plugged into the television board similar to CPU sockets (but with only 5ish pins in a circle)

These tubes were not particularly long-lived and were fragile physical devices. When they were "on the fritz" it was literally often possible to smack them back into place/alignment/operation. Hence the trope of a TV with a bad picture, slapping it around and voila it works again. This was a literal thing that really happened and works, at least until the internals of whatever tube were too far out of alignment.

At this point, rather than call an expensive repairman (always a man in those days), you could take your suspected bad tube to the grocery store, where there might be a machine that resembles a 1980s arcade cabinet, which has a bunch of various common vacuum-tube sockets on it. Dad will plug the 'bad' tube into the (in)correct socket and the machine will pronounce that tube to be GOOD or BAD with some version of accuracy.

With that information, dad can select a new identical or similar tube from the rack that's under the testing board, inside the cabinet.

Maybe it will work, maybe not.

Lots of specific tubes were replaceable with more generic versions that "will work" and there was a lot of effort to consolidate the vast number of tube variants, so another important tool was the equivalency chart-- look up your old tube in a book of tiny print/tables and see what generic part number might work to 'fix' the TV

Without having to call the repairman to your house, which was also very much a real thing.