As a user of a system, to access a file in Linux and UNIX, it is important that a user has the required permission for that specific file or directory. For example, as a regular user, perform cd into /root:

$ cd /root
bash: cd: /root/: Permission denied

We were not able to do so because of the permission denied error:

$ cd ~/

We were successfully able to do cd into the user's home directory because a user had the permission to access its own home directory.

Every file in UNIX or Linux has an owner and an associated group. It also has a set of permissions (read, write, and execute) with respect to the user, group, and others.

$ touch permission_test_file.txt    #  Creating a file
$ ls -l  permission_test_file.txt    # Seeing files' attributes
-rw-rw-r-- 1 foo foo 0 Aug 24 16:59 permission_test_file.txt