It is a command that does three things:

1. It starts to delete everything you have on the disk. That's the sudo rm -rf --no-preserve-root / part.
2. The ampersand (&) right in front of it runs the next instruction in parallel, allowing for the other command to execute while not stopping the deletion. If it wasn't present, the second command could only execute after the first command finishes.
3. The third part, :(){ :|:& };: is what's called a Fork Bomb. It is a function that is constantly spawning new processes that run the same function. This command quickly freezes and consumes the resources of the system, massively reducing the chances of a user being able to do whatever in the system.

Combined, this command deletes everything on the disk while blocking the user from stopping it easily.
Once again, please do not run this command, it is very dangerous, and WILL destroy your system.