chmod, setuid, setgid, sticky
chmod represents an octal representation of the read, write, and execute permissions of a directory or file.
Typical examples are 755 and 644, and chmod is also expressed in 4 octal digits.
The reason 3 characters are used is that if the first digit is not used, it is recognized as a default value of 0.
That is, 755 and 644 can be viewed as 0755 and 0644.
If the first byte is 0, it has no meaning, and if it is 1, it means sticky mode 2, setgid mode 4, setuid mode.