Apple has a built-in way to protect you from opening up potentially malicious apps on your computer in Mac OS X Lion, Mountain Lion, Mavericks, Yosemite, El Capitan, and macOS Sierra. This setting, named Gatekeeper, will never stop you from installing apps from the Mac App Store, but it could from anywhere else. If it's an app you're sure you want to install on your system, here's how to do it.