What is this button called on an iPod/iPhone and what's it for?

What is that button on the lower left screen for? It's blue since I pressed it when I took the screen shot… I have seen a few videos on youtube about an apple engineer who can unlock any iPod/iPhone by typing a bunch of digits fast…

The only buttons on the ipad/ipod/iphone that there's is the home button, power button, and the two volume buttons.

Maybe it clears all of the numbers you typed if you accidentally typed the wrong thing.

That button actually does not do anything.
Apple iOS products can't be unlocked by typing a lot of buttons really fast, a lock is a lock. The only way to remove a lock without knowing the code, is by reinstalling the entire device. Thereby effectively removing everything installed on the product at that time.
that way, you can always be assured, even if you lose your device, or it gets stolen, no one will have access to your details on there, because they will need to wipe it before being able to use it.