From LQWiki

Refers to a file or folder that cannot be modified. Examples include:

• shared documents on public webserver
• resources on a corporate Intranet
• config files

File or folders set as read-only mean that whilst anyone (with the correct permissions) may view it, no changes can be made to document that can be saved.

Depending on additional permissions such as the owner or group memberships, a user may be able to use the chmod or chgrp command to alter the attributes of a file, thus allowing them to perform read + write operations.