Alert Box
An alert box is used to ensure that the user of a web page has seen certain information. The user must press ok to show that they have understood/seen the information. Here's an example:
You should view the source for this page to see how the alert box is done using javascript. It's just below this sentence in the code.
For the assignment you need to explain what an alert box is and show an example that you find. It would be excellent if you were able to include an example.