You’re about to crawl inside a big dangerous machine to fix it, so you put a lock and tag on the power switch so that nobody can turn it on while you’re in there.

It's literally a lock on a switch to physically prevent it from being opened, because the equipment it's connected to is being worked on and energizing is dangerous

This is one of the most maliciously stupid things I've ever seen and that's saying something

Lock Out Tag Out. It's an OSHA safety procedure

Lock out/tag out

For when machinery is not to be turned on/energized because it's being maintained or is malfunctioning.

https://en.wikipedia.org/wiki/Lockout%E2%80%93tagout