what does the s stand for

[lina <lina.lastname@gmail.com>]

total 12
drwxr-xr-x 22 root root 4096 May 21  2011 linux-source-2.6.38-2-amd64-mm82
drwxr-xr-x 24 root root 4096 Jul 26 22:13 linux-source-3.0.0-mbp82-lina
drwxr-sr-x 24 root root 4096 Dec  1 17:35 linux-source-3.1-lina


Hi, How can I change the  -sr- to xr

and what does the s here stands for?

Thanks,