Linux is a multiuser operating system. In a multiuser environment, it is necessary to ensure that a user cannot access or modify files or directories that they arent supposed to. File permissions provide a protection mechanism for controlling access to files and directories.
Linux’s file security model is based on that of Unix. Each file or directory can be accessed or modified by the user who created it, or a group of users who have been given permission to do so. Permissions can also be defined for other users that do not belong to either of these two categories.
In this guide, we will go over how file permissions work in Linux for beginners. We’ll cover how you can view the permissions associated with files and directories and also how you can change them.
Source: Understanding Linux Permissions (Linux Academy)