Because of its association with security, a padlock is a common symbol in the world of computing, appearing as an icon in a wide range of applications. In most cases, the padlock indicates that a file or location is "locked" in some way, but the exact significance of this can vary.
Web browsers often use a padlock icon to indicate that the user is accessing a site over a secure socket layer (SSL) connection. In some cases, this is further elaborated with other symbols. For instance, in the Google Chrome browser, a green padlock means that the SSL connection is working successfully, while a yellow triangle on the padlock typically means that there is some insecure content on the page. A red x over the padlock means that the browser has detected dangerous insecure content or cannot verify the site's security certificate.
Windows 7 file system
In Windows 7, a gold-coloured lock icon appears at the bottom left of some folder icons. The presence of the lock icon means that someone, typically an administrator, has changed the permissions on this folder so that it cannot be accessed or modified. A list of which permissions have been denied can be seen by right-clicking on the folder, selecting "Properties" and clicking on the "Security" tab. A list of users appears; users can click on user names to see which actions are allowed or denied to each.
Mac OS security
A small padlock icon appears on the menu bar of some Apple computers. This icon is part of the "Keychain Access" application. Keychain Access allows the Mac OS X user to perform a range of security-related tasks, including locking the computer's screen. Clicking on the padlock icon will bring up a menu which allows the user to choose to lock the screen, lock or unlock other password-protected areas of the computer, or configure security preferences.
Other padlock icons
Padlock icons aren't limited to these applications; this simple image appears in a wide range of contexts. For instance, the Mozilla Firefox browser, which lost its padlock icon in more recent versions of the program, has an optional plug-in which restores the icon to the address bar. Some users even report that a blue and white padlock icon results from an error in the LCD computer monitor. Resetting the screen resolution should make the icon disappear.