Device rooting fails, but doesn't brick?
You can install Busybox from the market too: https://play.google.com/store/apps/details?id=stericson.busybox&amp;hl=en

And to your other question 


  i just wanna know what does busy box do? 


taken from market description: 

BusyBox is a software application that provides many standard Unix tools, much like the larger (but more capable) GNU Core Utilities. BusyBox is designed to be a small executable for use with the Linux kernel, which makes it ideal for use with embedded devices. It has been self-dubbed "The Swiss Army Knife of Embedded Linux".

The official BusyBox website about page has some more info about what it is too.