A method of limiting an application's access to system resources (sandbox) required for applications submitted to the Mac App Store and optional for non-AppStore applications.
Per Apple's email to Developer Center members:
As of June 1, 2012 all apps submitted to the Mac App Store must implement sandboxing. Enabling the default sandbox environment is as simple as checking the Enable Entitlements checkbox in Xcode target settings, allowing you to begin sandboxing your app.
Detailed information about App sandboxing can be found in Apple's Developer Center.