So what's a scripting language?
Basically a scripting language is a programming language that generally resides inside a host application and provides a sticky-gooey layer that makes it easy to automate otherwise boring and monotonous jobs and because it generally resides inside of a host application it has access to functions that mere mortal users don't.
Other examples of scripting languages include Visual Basic for Applications (VBA), Windows Scripting, VBScript and some might even argue that Perl is a scripting language (not me though, I just like to stir the pot).
In the case of AcroJS there's a great deal of power to be had with scripting the features and functions inside Acrobat. You can make mince meat of those tasks that you normally farm out to the junior in the office.
What's all this DOM business then?
Using the DOM, you gain access to the PDF document (and Acrobat) in a structured and object oriented fashion. A document object model basically sets out what objects exists in the document and also which features they provide to the programmer.
Where to from here?
OK, so you want to stamp your document. Maybe you need to give reviewers some advice about the document's status or sensitivity. This tip from author Ted Padova demonstrates how to add stamps with the Stamp Tool along with related comments.