Prompt Box
A prompt box is used to allow the visitor to your web page to input a value when they are loading your page. When the prompt box opens, the user is allowed to enter and value and then they must press either OK or Cancel to continue. If they click OK the box returns the value that the user entered but if they click Cancel they will returned to the original page and the retuen value of the box is nothing or null.
Here's an example, again you can view the code just under here in the source of this page.
For the assignment you need to explain what a prompt box is and show an example that you find. It would be excellent if you were able to include an example.