'chmod g+s' command

Hello I want to understand the role of the 'chmod g+s' command in unix.

I also would like to know what it does in this particular context :

cd /home/canard; touch un; chgrp canard .; chmod g+s .; touch deux ;

I understand all the commands roles except for 'chmod g+s' and I want to know the differences between the files 'un' and 'deux' resulting from this series of commands.

Thanks in advance, I'm a beginner in Unix.