Alert boxes are used to display some text information and have the visitor click on the OK button to continue on. This script is usually placed in the HEAD area of the code which makes the box appear before the actual page load.
ALERT creates the actual ALERT box to appear. Anything placed within the double quotes will become the message that appears on the ALERT box.
You can use it for in different instances as well such as putting it into a function then calling it when required.