That mean that any file dropped into the folder will take on the folder's owning group.
For example - suppose you have a folder called "shared" which belongs to user "intrpc" and group "users", and you drop a file into it, the copy will be belong to "intrpc" and "users" .
On most systems, if a directory's set-group-ID bit is set, newly created subfiles inherit the same group as the directory, and newly created subdirectories inherit the set-group-ID bit of the parent directory.
you can read about it here.
Why is it upper-cased (from the link you gave) ?
setgid has no effect if the group does not have execute permissions.
setgid is represented with a lower-case "s" in the output of ls. In
cases where it has no effect it is represented with an upper-case "S".