For apps that are downloaded from places other than the Mac App Store, 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 verify that apps haven’t been tampered with since they were signed.
Apple have announced some Gatekeeper changes for the forthcoming Mavericks 10.9.5 update to increase its functionality. This means that Apple will be reviewing all the current Apps and they will require to be re-approved.
This could mean some apps, that Gatekeeper previously allowed to run, will now generate a warning until their digital signatures have been updated. As a temporary work around, users can right-click the app and choose “Open” to bypass Gatekeeper for that application. Obviously only perform the action if you are 100% sure of the authenticity of the app.