The bang bang (!!) command is a shortcut to repeat the previous command you entered in your terminal. This command is very useful when you forget that you need admin rights to make a certain action, and lets you repeat it with super-user rights just by typing
sudo !!
instead of typing arrow-up, scrolling to the beginning of the line, adding sudo and hitting enter (imagine scrolling through those loooong apt-get commands). So many seconds gained! Yay!
There are many other bang-commands such as !x, !?x, !!:p and !!*. This blog post lists them and explains what they are for.
