Current versions of OS X have their Gatekeeper settings defaulted to allow applications from “Mac App Store and identified developers”. If you attempt to launch an app that isn’t signed by an identified developer, you’ll receive a Gatekeeper error.

Although Gatekeeper can be disabled in the Security system preference page, allowing any application to be launched, it is better to leave it set to “Mac App Store and identified developers” even if you need to run unsigned apps that you know are safe. The best practice is to control click on the app, choose Open from the contextual menu, then confirm that you want to run it. Going forward the app will continue to launch without error.