In the / directory, I use:
ls -alFd /tmp
to check the /tmp directory permissions and I get drwxrwxrwt.
I know rwxrwxrw means user, group, other permissions are read, write and execute.
But I don't know the meaning of the d and t. of the drwxrwxrwt, can someone explain it?
The d letter means it's a directory (a folder if you prefer that name). The t letter means that file is 'sticky'. Only the owner and root can delete a sticky file.
You may want to take a look at this page if you want to know more about the sticky file permission.
As explained in Understanding Linux File Permissions
Advanced Permissions
The special permissions flag can be marked with any of the following:
s is shown in the "executable" position. There's also S (capital), which means setuid / setgid but without the corresponding execute permission. Similarly there's T (capital), which means sticky without the corresponding execute permission. Then there's p, which means "named pipe".