You are trying to install or open an app that didn't come from the Mac App Store but you got a message saying App can't be opened because it is from an unidentified developer. This is how you bypass it.
Pretty simple and useful tutorial, where you will open a random photograph with smoke and convert it into something really not friendly for our environment.
The Popup Message behavior displays a JavaScript alert with the message you specify. In this lesson I will show you how to put an alert with a warning message on the website.