
Other than apps downloaded from the App Store, Gatekeeper also allows users to install apps from what they call identified developers. What if you trust an application developer but it is not in the App Store? Gatekeeper will still not let you install it – by default! You can change the settings and you will be able to do the installation in minutes. It is for your own security that macOS Mojave’s Gatekeeper attempts to keep such applications from being installed. This means that apps not in the Apple Store may or may not have reached this threshold.

In the Apple App Store are applications that Apple has reviewed and only those that are safe for use are included.

It is a security app made by Apple to guard what applications get in and which ones are kept out. Gatekeeper is just like the name suggests.
