What exactly does putting extern "C" into C++ code do?

For example:

extern "C" {
  void foo();
}

Today I've finally answered a question that's bothered me for a long time - if you've ever used a soldering iron, it FEELS like the smoke is ALWAYS blowing directly towards your face, but for years I wrote it off as confirmation bias. It's not. There's a real physical reason the smoke is funneled annoyingly towards your eyeballs, and in this video, aided by a plane laser and a smoke machine, I figure it out!

CORRECTIONS:
1: Safety! Turns out mineral oil smoke CAN be toxic and produce Lipoid Pneumonia, thanks to Cheetah Spottycat below for pointing this out. I chose it initially because the internet told me it wouldn't leave residue all over my garage, but I cannot endorse it as a try-this-at-home technique!

USEFUL DISCUSSIONS:
https://www.reddit.com/r/electronics/comments/gsfayl/its_not_just_confirmation_bias_solder_smoke/
https://hackaday.com/2020/06/01/why-does-solder-smoke-always-find-your-face/

GIFS:
https://gfycat.com/querulousunsightlybrownbear
https://gfycat.com/easyindelibledegus
https://gfycat.co…