Note: This article applies to Macintosh OS X 10.10 Yosemite versions and earlier. The repair permissions tool was removed from OS X 10.11 El Capitan, macOS 10.12 Sierra and later versions.
At times, we may ask you to run "Repair Permissions" on your Mac. Often, this will fix a host of issues, For example, even though you may be an Administrator on your Mac, you may be having difficulty running an installer because of a permissions issue with the destination folder. This tool is part of the Disc Utility application, which is located in the Utilities folder, inside the Applications folder on your Macintosh hard drive.
In Finder go to: Applications/Utilities/Disk Utility
When you launch the Disk Utility you will see a window like the one below:
Just select your Hard Drive, and click the "Repair Disk Permissions" button. It may take a while to finish but often this will fix many issues with your Mac.