umask

    Permission | Binary | Octal  | User  | Group | Other |
    ======================================================
      r        |  100   |   4    |       |       |       |
      w        |  010   |   2    |       |       |       |
      x        |  001   |   1    |       |       |       |
    =======================================================

for example

• the owner of file has read, write, and execute
• file group only read and execute
• the other only can read

the permission is like this

u   g   o
rwx r-x  r--

aka, because rwx has total point of 4 + 2 + 1, so the permission for user is 7 because group only have 4 & 1, so the permission is 5 and last. other only read, which is 4

the equalivalent command is chmod 754