The “s” is a “sticky bit” which sets that property for all subsequently created subdirs and/or files. Depending on where the “s” is dictates if it’s “suid” or “sgid”
‘man chmod’ will give more info
1755 = drwxr-xr-t 2755 = drwxr-sr-x 3755 = drwxr-sr-t 4755 = drwsr-xr-x 5755 = drwsr-xr-t 6755 = drwsr-sr-x 7755 = drwsr-sr-t