You just need to give your Mac the equivalent of a special sign, an indication that you know what you’re doing and you didn’t just click on this app accidentally. You don’t need to turn off Gatekeeper to launch a new app, though. (Even if you have Gatekeeper turned off, Quarantine will still warn you the first time you open an app you’ve downloaded from the Internet.) The very first time you try to launch it, Quarantine goes to work-and if it fails Gatekeeper’s tests (it’s not from the Mac App Store or not signed by an Apple-recognized developer), Gatekeeper will warn you or outright refuse to launch it. When you download executable software to your Mac, it’s tagged automatically by a system called Quarantine, which sets a special flag on the file.

If you don’t want to ever be warned about anything, just set the Allow Apps Downloaded From preference to “Anywhere.” I like keeping that setting on, but sometimes I just want to launch an app I’ve downloaded, and Gatekeeper gets in the way. You can configure Gatekeeper from your Mac’s System Preferences app, in the General tab of the Security & Privacy preference pane. It’s sort of a middle ground between the tightly controlled environment of the App Store, in which every App is approved by an Apple reviewer, and the Wild West of classic Mac software development, in which your Mac has no idea where the code it’s running has been.

With OS X Mountain Lion, Apple introduced Gatekeeper, a feature that seeks to make Mac software more secure by warning you if software you use is by a developer who hasn’t registered with Apple. Note: This story has not been updated since 2016. Quick Tip: Override Gatekeeper with one click

Mac gatekeeper settings