It allows to ask the user something. It has no actions or conditions, just an expression.
You call it from a variable or a text object like if it were a variable, using prompt.p("Text you want to show in the prompt") and it will work.
Download the plugin
Download a .capx example
Try it in web
Edit: Improve in the plugin, now you can select the text you want to show in the prompt window.
Thanks, i hope it will usefull.