".dirty" suffix when building git from the source code

Hi all,

I have cloned the git repository and built the v1.6.4.4 tag. The
compilation went OK and I got the tool up and running. However, when I
generate patches, I see that the tool comments the patches with
"1.6.4.4.dirty".

My question is: what is the meaning of the ".dirty" suffix?

Thanks in advance.

Best Regards,
Mohamed