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I know what it means for a file to have suid permission. It means when other users have execute permission for it, they execute as the owner of the file. But what does it imply when a folder has suid permission? I did some testing and it seems nothing special for the folder. Could anyone help to plain a little? Thanks.

I'm using Oracle Linux 7.6.

root:[~]# cat /etc/*release*
Oracle Linux Server release 7.6
NAME="Oracle Linux Server"
VERSION="7.6"
ID="ol"
VARIANT="Server"
VARIANT_ID="server"
VERSION_ID="7.6"
PRETTY_NAME="Oracle Linux Server 7.6"
ANSI_COLOR="0;31"
CPE_NAME="cpe:/o:oracle:linux:7:6:server"
HOME_URL="https://linux.oracle.com/"
BUG_REPORT_URL="https://bugzilla.oracle.com/"

ORACLE_BUGZILLA_PRODUCT="Oracle Linux 7"
ORACLE_BUGZILLA_PRODUCT_VERSION=7.6
ORACLE_SUPPORT_PRODUCT="Oracle Linux"
ORACLE_SUPPORT_PRODUCT_VERSION=7.6
Red Hat Enterprise Linux Server release 7.6 (Maipo)
Oracle Linux Server release 7.6
cpe:/o:oracle:linux:7:6:server
root:[~]#

Below is my testing on a freshly installed server.

root:[~]# pwd
/root
root:[~]# ls -lad /root
dr-xr-x---. 9 root root 4096 Aug 16 22:07 /root
root:[~]# mkdir test
root:[~]# ls -lad test
drwxr-xr-x. 2 root root 4096 Aug 16 22:07 test
root:[~]#
root:[~]# useradd a
root:[~]# passwd a
Changing password for user a.
New password:
BAD PASSWORD: The password is a palindrome
Retype new password:
passwd: all authentication tokens updated successfully.
root:[~]# chmod u+s test
root:[~]#
root:[~]# su - a
[a@localhost ~]$ cd /root/test
-bash: cd: /root/test: Permission denied
[a@localhost ~]$ cd /root
-bash: cd: /root: Permission denied
[a@localhost ~]$ logout
root:[~]#
root:[~]# ls -lad /root
dr-xr-x---. 10 root root 4096 Aug 16 22:07 /root
root:[~]# chmod o+x /root
root:[~]#
root:[~]# su - a
Last login: Fri Aug 16 22:08:54 CST 2019 on pts/0
[a@localhost ~]$ cd /root/test
[a@localhost test]$
[a@localhost test]$ pwd
/root/test
[a@localhost test]$ ls -la .
total 8
drwsr-xr-x. 2 root root 4096 Aug 16 22:07 .
dr-xr-x--x. 10 root root 4096 Aug 16 22:07 ..
[a@localhost test]$ touch file1
touch: cannot touch ‘file1’: Permission denied
[a@localhost test]$ logout
root:[~]#
root:[~]# chmod o+w test/
root:[~]#
root:[~]# su - a
Last login: Fri Aug 16 22:09:31 CST 2019 on pts/0
[a@localhost ~]$
[a@localhost ~]$ cd /root/test
[a@localhost test]$ touch file1
[a@localhost test]$ ls -la
total 8
drwsr-xrwx. 2 root root 4096 Aug 16 22:11 .
dr-xr-x--x. 10 root root 4096 Aug 16 22:07 ..
-rw-rw-r--. 1 a a 0 Aug 16 22:11 file1
[a@localhost test]$ mkdir folder1
[a@localhost test]$ ls -la
total 12
drwsr-xrwx. 3 root root 4096 Aug 16 22:11 .
dr-xr-x--x. 10 root root 4096 Aug 16 22:07 ..
-rw-rw-r--. 1 a a 0 Aug 16 22:11 file1
drwxrwxr-x. 2 a a 4096 Aug 16 22:11 folder1
[a@localhost test]$

As you can see, it seems the files and folders the user a created in /root/test didn't inherit the owner and group of it. The owner and group is a and not root. Are there any problems with my testing? I'm new in Linux.

According to the GNU manual, it means files (including subfolders) created in the directory will inherit its group and user:

On a few systems, a directory’s set-user-ID bit has a similar effect on the ownership of new subfiles and the set-user-ID bits of new subdirectories. These mechanisms let users share files more easily, by lessening the need to use chmod or chown to share new files.