You can see file permissions in linux by typing

$ ls -l

You can think of permissions as a set of bits, and then each 3 bits as an octal digit. eg

rwx r-x r-x
111 101 101
  7   5  5

You can use the chmod command to set the permissions of a file or directory using the desired 3 digit octal code. eg.

$ chmod 700 f.txt