[Typo3] How can I develop an existing extension?I've "difference between the originally installed version and the current was detected!"

[M Okhovvat]

Hi 
 I want to modify an exisiting extension.when I change it and click on
save file ,it writes in the up of the save file button some MD5 codes
from previous file and saved file that they are different.
 
And when in extmanager I select that changed extension and select
Information from drop down menu in the up right of the extmanager page
, in the bottom of the page this message is wroten :
"difference between the originally installed version and the current
was detected!"
and writes name of some files of extension that I've been modified.

What is this MD5 for?
 Am I allowed to modify and develop an existing extension?
Can I modify files of an extension in my favorite editor out of
extmanager\edit and save them?
What I can do with this MD5? This is MD5 of wich part of extension files? Can I
modify existing file and then replace its MD5 by MD5 of changed
extension my self (without using extmanager)?

Thanks.