'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.