From the top

sudo

Is a super-user command. Anything after "sudo" runs as the linux equivalent of an administrator. "apt-get" is the 'package manager' for Debian, and Debian based linux distrobutions (like Ubuntu). It's how most linux users install the majority of their software. So:

sudo apt-get [install] rekt!

Essentially means, "As root, use the Advanced Packaging Tool to procure (get) the package 'rekt!' from the official repositories".

edit: As darkliquid0 stated below, that command would not function without clarification. I use pacman instead of apt, so I'll do more research next time.

rm

is the remove command. The "-r" flag means recursive, essentially allowing it to delete directories and their contents. The "-f" flag means force. No dialogs, no interactivity, and no questions.

Finally, 'rm' takes a file or directory. In this case being '/' or root.

rm / -rf

So this one means "Delete everything (files and folders) on the root filesystem without asking the user any questions". I don't think it would work without 'sudo' or the user running as root off the bat. The command typed would destroy the main filesystem of many *nix operating systems.