What are permissions?
Every file and folder on a Mac OS X hard drive has a set of permissions settings that determine which user(s) have access to each item, and exactly what that access is. For example, permissions dictate whether or not a particular user can open and edit a particular file. But permissions also determine which items the operating systemor specific parts of itcan access and modify, and which files are accessible by applications. (Brian Tanaka offers more details about various types of permissions in this excerpt from his Take Control of Permissions in Mac OS X ebook .)
What does repairing permissions do?
The Repair Disk Permissions functionthe process that actually performs the task of repairing permissionsexamines certain files and folders on your Macs hard drive to see if their current permissions settings are what Mac OS X expects them to be; if discrepancies are found, the offending permissions are changed to match the expected settings.
(In Mac OS X 10.3 and later, repairing permissions also performs one other, unrelated, task: If the invisible /tmp symbolic linkwhich is linked to the /private/tmp directoryis missing, the link will be recreated.)
Why is it necessary to repair permissions?
If permissions on particular files are incorrecti.e., not what Mac OS X expects them to be or not what they need to be for your Macs normal operationyou can experience problems when the operating system tries to access or modify those files. For example, you may have trouble logging in to your account, printing, launching applications, or even starting up your Mac. Similarly, if an applicationfrom Apple or a third-party developerneeds access to a particular file or folder to function, and the permissions on that item have changed in a way that prevents such access, the application may not function properly (or at all). The Repair Disk Permissions function can fix such problems by ensuring that certain files have the correct permissions.
Theres also a security element here: Many system-level files have permissions set a particular way so that applications or users that shouldnt be meddling with those files cant. If the permissions on certain system-level files somehow get changed so that access to those files is no longer restricted, youve got the potential for a major security issue. Repairing permissions can resolve such issues by resetting permissions on those files to prevent unauthorized access.
Source:
http://www.macworld.com/article/52220/2006/08/repairpermissions.html