Gatekeeper, a security feature in Mountain Lion that only allows certain applications to be opened. The default setting is to allow only Mac App Store and trusted developer applications
Gatekeeper makes downloading apps safer. Gatekeeper makes it safer to download apps by protecting you from inadvertently installing malicious software on your Mac. The safest place to download apps for your Mac is the Mac App Store. Apple reviews each app before it’s accepted by the store, and if there’s ever a problem with an app, Apple can quickly remove it from the store. When you download software from any other place on the Internet, Gatekeeper makes that safer, too. Developers can get a unique Developer ID from Apple and use it to digitally sign their apps. The Developer ID allows Gatekeeper to block apps created by malware developers and to verify that apps haven’t been tampered with. If an app was developed by an unknown developer — one with no Developer ID — Gatekeeper can keep your Mac safe by blocking the app from being installed.