Occasionally, you've typed a long command to bash; and, before you hit return, notice that there was a spelling mistake early in the line. You could just delete all the way back and retype everything you need to, but that takes much too much effort! Instead, you can use the arrow keys to move back there, delete the bad character or two, and type the correct information.
There are many special keys to help you edit your command line, most of
them similar to the commands used in GNU Emacs;. For
instance, 
flips two adjacent characters.
You'll be able to find most of the commands in the
chapter on Emacs, Chapter 
.