We will be focused on permissions on files and directories, the building blocks of a web site. Permissions can control only three different actions:
The ability to read a file, or get a directory listing.
The ability to write to a file, or the ability to create, rename and delete files and subdirectories on a directory.
For files, it controls the ability to be directly executable from the command line. It is only meaningful for binary programs and executable scripts. For directories, it controls the ability to change to that directory. Note that if this is disabled you can't usually obtain a directory listing and file read operations might fail.
These three actions, combined with the three access request groups (owning user, owning group and the rest of the world) give us a total of nine distinct operations which can be controlled. Each action is an on/off switch. If a permission is set, it is turned on and the right to perform the action is granted. If the permission is not set, the switch is off and the right to perform the action is not granted.