It's a running joke. https://en.wikipedia.org/wiki/Magic_smoke

Also fun is looking at all the components that have released their magic.

Dave Jones has a good video on it (well series to a degree): https://www.youtube.com/watch?v=SoeVNaz7KGc

The Jargon File.

Magic smoke is injected into electronic components at the factory, in order to make them work. If you let the smoke escape the package, your part has failed permanently. But as long as the smoke remains inside, there is still a chance you can repair any failure by cooling the part off for a while.